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THE  TELESCOPE 


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Galileo's  Telescopes. 
(Bull,  de  la  Soc.  Astron.  de  France.) 


{Frontispiece) 


THE  TELESCOPE 


LOUI&^BELL,  Ph.D. 


CONSULTING    engineer;   FELLOW,    AMERICAN   ACADEMY   OF   ABTS   &    SCIENCES;    PAST- 

PKESIDENT,    THE    ILLUMINATING   ENGINEERING   SOCIETY;   MEMBER, 

AMERICAN   ASTRONOMICAL  SOCIETY 


fiOST02^  COL-  h 


First  Edition 


McGRAW-HILL  BOOK  COMPANY,  Inc. 
NEW  YORK:  370  SEVENTH  AVENUE 

LONDON:  6  &  8  BOUVERIE  ST.,  E.  C.  4 

1922 


Copyright,  1922,  by  the 
McGraw-Hill  Book  Company,  Inc. 


Q3  ^3r 


1 79201 


THE     M.A.FIj'E    FHKSS     X  O  R  K    PA 


PREFACE 

This  book  is  written  for  the  many  observers,  who  use  tele- 
scopes for  study  or  pleasure  and  desire  more  information  about 
their  construction  and  properties.  Not  being  a  "handbook"  in 
two  or  more  thick  quartos,  it  attempts  neither  exhaustive  tech- 
nicalities nor  popular  descriptions  of  great  observatories  and  their 
work.  It  deals  primarily  with  principles  and  their  application  to 
such  instruments  as  are  likely  to  come  into  the  possession,  or 
within  reach,  of  students  and  others  for  whom  the  Heavens  have 
a  compelling  call. 

Much  has  been  written  of  telescopes,  first  and  last,  but  it  is  for 
the  most  part  scattered  through  papers  in  three  or  four  languages, 
and  quite  inaccessible  to  the  ordinary  reader.  For  his  benefit  the 
references  are,  so  far  as  is  practicable,  to  English  sources,  and 
dimensions  are  given,  regretfully,  in  English  units.  Certain 
branches  of  the  subject  are  not  here  discussed  for  lack  of  space 
or  because  there  is  recent  literature  at  hand  to  which  reference 
can  be  made.  Such  topics  are  telescopes  notable  chiefly  for  their 
dimensions,  and  photographic  apparatus  on  which  special  treatises 
are  available. 

Celestial  photography  is  a  branch  of  astronomy  which  stands 
on  its  own  feet,  and  although  many  telescopes  are  successfully 
used  for  photography  through  the  help  of  color  screens,  the 
photographic  telescope  proper  and  its  use  belongs  to  a  field 
somewhat  apart,  requiring  a  technique  quite  its  own. 

It  is  many  years,  however,  since  any  book  has  dealt  with  the 
telescope  itself,  apart  from  the  often  repeated  accounts  of  the 
marvels  it  discloses.  The  present  volume  contains  neither  pic- 
tures of  nebulae  nor  speculations  as  to  the  habitibility  of  the 
planets;  it  merely  attempts  to  bring  the  facts  regarding  the 
astronomer's  chief  instrument  of  research  somewhere  within 
grasp  and  up  to  the  present  time. 

The  author  cordially  acknowledges  his  obligations  to  the 
important  astronomical  journals,  particularly  the  Astro-physical 
Journal,  and  Popular  Astronomy  in  this  country;  The  Observa- 
tory, and  the  publications  of  the  Royal  Astronomical  Society 


viii  PREFACE 

in  England;  the  Bulletin  de  la  Societe  Astronomique  de  France; 
and  the  Astronomische  Nachrichten;  which,  with  a  few  other  jour- 
nals and  the  official  reports  of  observatories  form  the  body  of 
astronomical  knowledge.  He  also  acknowledges  the  kindness  of 
the  various  publishers  who  have  extended  the  courtesy  of  illus- 
trations, especially  Macmillan  &  Co.  and  the  Clarendon  Press, 
and  above  all  renders  thanks  to  the  many  friends  who  have 
cordially  lent  a  helping  hand — the  Director  and  staff  of  the 
Harvard  Observatory,  Dr.  George  E.  Hale,  C.  A.  R.  Lundin, 
manager  of  the  Alvan  Clark  Corporation,  J.  B.  McDowell,  suc- 
cessor of  the  Brashear  Company,  J,  E.  Bennett,  the  American 
representative  of  Carl  Zeiss,  Jena,  and  not  a  few  others. 

Louis  Bell. 

Boston,  Mass., 

February,  1922. 


CONTENTS 

Page 

Preface vii 

Chap. 

I.  The  Evolution  of  the  Telescope 1 

II.  The  Modern  Telescope 31 

III.  Optical  Glass  and  Its  Working 57 

IV.  The  Properties  of  Objectives  and  Mirrors 76 

V.  Mountings 98 

VI.  Eye-pieces 134 

VII.  Hand  Telescopes  and  Binoculars 150 

VIII.  Accessories 165 

IX.  The  Testing  and  Care  of  Telescopes 201 

X.  Setting  up  and  Housing  the  Telescope 228 

XI.  Seeing  and  Magnification 253 

Appendix 279 

Index 281 


IX 


THE  TELESCOPE 

CHAPTER  I 
THE  EVOLUTION  OF  THE  TELESCOPE 

In  the  credulous  twaddle  of  an  essay  on  the  Lost  Arts  one  may 
generally  find  the  telescope  ascribed  to  far  antiquity.  In  place 
of  evidence  there  is  vague  allusion  of  classical  times  or  wild 
flights  of  fancy  like  one  which  argued  from  the  Scriptural  state- 
ment that  Satan  took  up  Christ  into  a  high  mountain  and 
showed  him  all  the  kingdoms  of  the  earth,  that  the  Devil  had  a 
telescope — bad  optics  and  worse  theology. 

In  point  of  fact  there  is  not  any  indication  that  either  in  clas- 
sical times,  or  in  the  black  thousand  years  of  hopeless  ignorance 
that  followed  the  fall  of  Roman  civilization,  was  there  any 
knowledge  of  optical  instruments  worth  mentioning. 

The  peoples  that  tended  their  flocks  by  night  in  the  East  alone 
kept  alive  the  knowledge  of  astronomy,  and  very  gradually,  with 
the  revival  of  learning,  came  the  spirit  of  experiment  that  led 
to  the  invention  of  aids  to  man's  natural  powers. 

The  lineage  of  the  telescope  runs  unmistakably  back  to 
spectacles,  and  these  have  an  honorable  history  extending  over 
more  than  six  centuries  to  the  early  and  fruitful  days  of  the 
Renaissance. 

That  their  origin  was  in  Italy  near  the  end  of  the  thirteenth 
century  admits  of  little  doubt.  A  Florentine  manuscript  letter 
of  1289  refers  to  ''Those  glasses  they  call  spectacles,  lately 
invented,  to  the  great  advantage  of  poor  old  men  when  their 
sight  grows  weak,"  and  in  1305  Giordano  da  Rivalto  refers  to 
them  as  dating  back  about  twenty  years. 

Finally,  in  the  church  of  Santa  Maria  Maggiore  in  Florence  lay 
buried  Sal  vino  d'Amarto  degli  Armati,  (obiit  1317)  under  an 
epitaph,  now  disappeared,  ascribing  to  him  the  invention  of 
spectacles.  W.  B.  Carpenter,  F.  R.  S.,  states  that  the  inventor 
tried  to  keep  the  valuable  secret  to  himself,  but  it  was  discovered 
and  published  before  his  death.  At  all  events  the  discovery 
moved  swiftly.     By  the  early  fourteenth  century  it  had  spread  to 

1 


2  THE  TELESCOPE 

the  Low  Countries  where  it  was  destined  to  lead  to  great  results, 
and  presently  was  common  knowledge  over  all  civilized  Europe. 

It  was  three  hundred  years,  however,  between  spectacles 
and  the  combination  of  spectacle  lenses  into  a  telescope,  a  lapse 
of  time  which  to  some  investigators  has  seemed  altogether 
mysterious.  The  ophthalmological  facts  lead  to  a  simple  expla- 
nation. The  first  spectacles  were  for  the  relief  of  presbyopia,  the 
common  and  lamentable  affection  of  advancing  years,  and  for 
this  purpose  convex  lenses  of  very  moderate  power  sufficed,  nor 
was  material  variation  in  power  necessary.  Glasses  having  a 
uniform  focus  of  a  foot  and  a  half  or  thereabouts  would  serve 
every  practical  purpose,  but  would  be  no  material  for  telescopes. 

Myopia  was  little  known,  its  acquired  form  being  rare  in  a 
period  of  general  illiteracy,  and  glasses  for  its  correction,  espe- 
cially as  regards  its  higher  degrees,  probably  came  slowly  and 
were  in  very  small  demand,  so  that  the  chance  of  an  optical 
craftsman  having  in  hand  the  ordinary  convex  lenses  and  those 
of  strong  negative  curvature  was  altogether  remote.  Indeed  it 
was  only  in  1575  that  Maurolycus  published  a  clear  description 
of  myopia  and  hypermetropia  with  the  appropriate  treatment  by 
the  use  of  concave  and  convex  lenses.  Until  both  of  these,  in 
quite  various  powers,  were  available,  there  was  small  chance  of 
hitting  upon  an  instrument  that  required  their  use  in  a  highly 
special  combination. 

At  all  events  there  is  no  definite  trace  of  the  discovery  of 
telescopic  vision  until  1608  and  the  inventor  of  record  is 
unquestionably  one  Jan  Lippershey,  a  spectacle  maker  of  Middel- 
burg  in  Zeeland,  a  native  of  Wesel.  On  Oct.  2,  1608  the  States- 
General  took  under  consideration  a  petition  which  had  been 
presented  by  Lippershey  for  a  30-year  patent  to  the  exclusive 
right  of  manufacture  of  an  instrument  for  seeing  at  a  distance,  or 
for  a  suitable  pension,  under  the  condition  that  he  should  make 
the  instrument  only  for  his  country's  service. 

The  States  General  pricked  up  its  ears  and  promptly  appointed 
on  Oct.  4  a  committee  to  test  the  new  instrument  from  a  tower  of 
Prince  Maurice's  palace,  allotting  900  florins  for  the  purchase  of 
the  invention  should  it  prove  good.  On  the  6th  the  committee 
reported  favorably  and  the  Assembly  agreed  to  give  Lippershey 
900  florins  for  his  instrument,  but  desired  that  it  be  arranged  for 
use  with  both  eyes. 

Lippershey  therefore  pushed  forward  t^o  the  binocular  form  and 


THE  EVOLUTION  OF  THE  TELESCOPE 


two  months  later,  Dec.  9,  he  announced  his  success.  On  the 
15th  the  new  instrument  was  examined  and  pronounced  good, 
and  the  Assembly  ordered  two  more  binoculars,  of  rock  crystal, 
at  the  same  price.  They  denied  a  patent  on  the  ground  that  the 
invention  was  known  to  others,  but  paid  Lippershey  liberally  as  a 
sort  of  retainer  to  secure  his  exclusive  services  to  the  State.  In 
fact  even  the  French  Ambassador,  wishing  to  obtain  an  instru- 
ment from  him  for  his  King,  had  to  secure  the  necessary  author- 
ization from  the  States-General. 


Bull,  de  la  Soc.  Astroii.  de  France. 
Fig.  1. — Jan  Lippershey,  Inventor  of  the  Telescope. 

It  is  here  pertinent  to  enquire  what  manner  of  optic  tube 
Lippershey  showed  to  back  up  his  petition,  and  how  it  had  come 
to  public  knowledge.  As  nearly  as  we  may  know  these  first  tele- 
scopes were  about  a  foot  and  a  half  long,  as  noted  by  Huygens,  and 
probably  an  inch  and  a  half  or  less  in  aperture,  being  constructed 
of  an  ordinary  convex  lens  such  as  was  used  in  spectacles  for  the 
aged,  and  of  a  concave  glass  suitable  for  a  bad  case  of  short 
sightedness,  the  only  kind  in  that  day  likely  to  receive  attention. 


4  THE  TELESCOPE 

It  probably  magnified  no  more  than  three  or  four  diameters 
and  was  most  Hkely  in  a  substantial  tube  of  firmly  rolled,  glued, 
and  varnished  paper,  originally  without  provision  for  focussing, 
since  with  an  eye  lens  of  rather  low  power  the  need  of  adjustment 
would  not  be  acute. 

As  to  the  invention  being  generally  known,  the  only  definite 
attempt  to  dispute  priority  was  made  by  James  Metius  of 
Alkmaar,  who,  learning  of  Lippershey's  petition,  on  Oct.  17, 1608, 
filed  a  similar  one,  alleging  that  through  study  and  labor  extend- 
ing over  a  couple  of  years  he,  having  accidentally  hit  upon  the 
idea,  had  so  far  carried  it  out  that  his  instrument  made  distant 
objects  as  distinct  as  the  one  lately  offered  to  the  States  by  a 
citizen  and  spectacle  maker  of  Middelburg. 

He  apparently  did  not  submit  an  instrument,  was  politely 
told  to  perfect  his  invention  before  his  petition  was  further  con- 
sidered, and  thereafter  disappears  from  the  scene,  whatever  his 
merits.  If  he  had  actually  noted  telescopic  vision  he  had  neither 
appreciated  its  enormous  importance  nor  laid  the  facts  before 
others  who  might  have  done  so. 

The  only  other  contemporary  for  whom  claims  have  been  made 
is  Zacharius  Jansen,  also  a  spectacle  maker  of  Middelburg,  to 
whom  Pierre  Borel,  on  entirely  second  hand  information,  ascribed 
the  discovery  of  the  telescope.  But  Borel  wrote  nearly  fifty 
years  later,  after  all  the  principals  were  dead,  and  the  evidence 
he  collected  from  the  precarious  memories  of  venerable  witnesses 
is  very  conflicting  and  points  to  about  1610  as  the  date  when 
Jansen  was  making  telescopes — like  many  other  spectacle 
makers.^ 

Borel  also  gave  credence  to  a  tale  that  Metius,  seeking 
Jansen,  strayed  into  Lippershey's  shop  and  by  his  inquiries  gave 
the  shrewd  proprietor  his  first  hint  of  the  telescope,  but  set  the 
date  at  1610.  A  variation  of  this  tale  of  the  mysterious  stranger, 
due  to  Hieronymus  Sirturus,  contains  the  interesting  intimation 
that  he  may  have  been  of  supernatural  origin — not  further  speci- 
fied. There  are  also  the  reports,  common  among  the  ignorant 
or  envious,  that  Lippershey's  discovery  was  accidental,  even 
perhaps  made  by  his  children  or  apprentice. ' 

Just  how  it  actually  was  made  we  do  not  know,  but  there 
is  no  reason  to  suppose  that  it  was  not  in  the  commonplace  way 

1  There  is  a  very  strong  probability  that  Jansen  was  the  inventor  of  the 
compound  microscope  about  the  beginning  of  the  seventeenth  century. 


THE  EVOLUTION  OF  THE  TELESCOPE  5 

of  experimenting  with  and  testing  lenses  that  he  had  produced, 
perhaps  those  made  to  meet  a  vicious  case  of  myopia  in  one  of 
his  patrons. 

When  the  discovery  was  made  is  somewhat  clearer.  Plainly 
it  antedated  Oct.  2,  and  in  Lippershey's  petition  is  a  definite 
statement  that  an  instrument  had  already  been  tested  by  some, 
at  least,  of  the  members  of  the  States-General.  A  somewhat 
vague  and  gossipy  note  in  the  Mercure  Frangaise  intimates  that 
one  was  presented  to  Prince  Maurice  "about  September  of  the 
past  year"  (1608)  and  that  it  was  shown  to  the  Council  of  State 
and  to  others. 

Allowing  a  reasonable  time  between  Lippershey's  discovery 
and  the  actual  production  of  an  example  suitable  for  exhibition 
to  the  authorities,  it  seems  likely  that  the  invention  dates  back 
certainly  into  the  summer  of  1608,  perhaps  even  earlier. 

At  all  events  there  is  every  indication  that  the  news  of  it 
spread  like  wild-fire.  Unless  Lippershey  were  unusually  careful 
in  keeping  his  secret,  and  there  are  traditions  that  he  was  not, 
the  sensational  discovery  would  have  been  quickly  known  in  the 
little  town  and  every  spectacle  maker  whose  ears  it  reached  would 
have  been  busy  with  it. 

If  the  dates  given  by  Simon  Marius  in  his  Mundus  Jovialis 
be  correct,  a  Belgian  with  an  air  of  mystery  and  a  glass  of  which 
one  of  the  lenses  was  cracked,  turned  up  at  the  Frankfort  fair 
in  the  autumn  of  1608  and  at  last  allowed  Fuchs,  a  nobleman  of 
Bimbach,  to  look  through  the  instrument.  Fuchs  noted  that  it 
magnified  "several"  times,  but  fell  out  with  the  Belgian  over  the 
price,  and  returning,  took  up  the  matter  with  Marius,  fathomed 
the  construction,  tried  it  with  glasses  from  spectacles,  attempted 
to  get  a  convex  lens  of  longer  focus  from  a  Nuremburg  maker, 
who  had  no  suitable  tools,  and  the  following  summer  got  a  fairly 
good  glass  from  Belgium  where  such  were  already  becoming 
common. 

With  this  Marius  eventually  picked  up  three  satellites  of 
Jupiter — the  fourth  awaited  the  arrival  of  a  superior  telescope 
from  Venice.  Early  in  1609  telescopes  "about  a  foot  long" 
were  certainly  for  sale  in  Paris,  a  Frenchman  had  offered  one  in 
Milan  by  May  of  that  year,  a  couple  of  months  later  one  was  in 
use  by  Harriot  in  England,  an  example  had  reached  Cardinal 
Borghese,  and  specimens  are  said  to  have  reached  Padua.  Fig.  2 
from  the  "Mundus  Jovialis,"  shows  Marius  with  his  "Perspicil- 


6 


THE  TELESCOPE 


ium,"  the  first  published  picture  of  the  new  instrument.  Early 
in  1610  telescopes  were  being  made  in  England,  but  if  the  few 
reports  of  performance,  even  at  this  date,  are  trustworthy,  the 
"Dutch  trunk"  of  that  period  was  of  very  indifferent  quality  and 
power,  far  from  being  an  astronomical  instrument. 

One  cannot  lay  aside  this  preliminary  phase  of  the  evolution 
of  the  telescope  without  reference  to  the  alleged  descriptions  of 
telescopic  apparatus  by  Roger  Bacon,   (c.  1270),  Giambattista 


The  Observatory. 
Fig.  2. — Simon  Marius  and  his  Telescope. 

della  Porta  (1558),  and  Leonard  Digges  (1571),  details  of  which 
may  be  found  in  Grant's  History  of  Physical  Astronomy  and  many 
other  works. 

Of  these  the  first  on  careful  reading  conveys  strongly  the  con- 
viction that  the  author  had  a  pretty  clear  idea  of  refraction  from 
the  standpoint  of  visual  angle,  yet  without  giving  any  evidence 
of  practical  acquaintance  with  actual  apparatus  for  doing  the 
things  which  he  suggests. 

Given  a  suitable  supply  of  lenses,  it  is  reasonably  certain  that 
Bacon  was  clever  enough  to  have  devised  both  telescope  and 


THE  EVOLUTION  OF  THE  TELESCOPE 


microscope,  but  there  is  no  evidence  that  he  did  so,  although  his 
manifold  activities  kept  him  constantly  in  public  view.  It  does 
not  seem  unlikely,  however,  that  his  suggestions  in  manuscripts, 
quite  available  at  the  time,  may  have  led  to  the  contemporaneous 
invention  of  spectacles. 

Porta's  comments  sound  like  an  echo  of  Bacon's,  plus  a  rather 
muddled  attempt  to  imagine  the  corresponding  apparatus. 
Kepler,  certainly  competent  and  familiar  with  the  principles 
of  the  telescope,  found  his  description  entirely  unintelhgible. 
Porta,  however,  was  one  of  the  earliest  workers  on  the  camera 
obscura  and  upon  this  some  of  his  cryptic  statements  may  have 
borne. 

Somewhat  similar  is  the  situation  respecting  Digges.  His  son 
makes  reference  to  a  Ms.  of  Roger  Bacon  as  the  source  of  the 
marvels  he  describes.  The  whole  account,  however,  strongly 
suggests  experiments  with  the  camera  obscura  rather  than  with  the 
telescope. 

The  most  that  ca'n  be  said  with  reference  to  any  of  the  three 
is  that,  if  he  by  any  chance  fell  upon  the  combination  of  lenses 
that  gave  telescopic  vision,  he  failed  to  set  down  the  facts  in  any 
form  that  could  be  or  was  of  use  to  others.  There  is  no  reason 
to  believe  that  the  Dutch  discovery,  important  as  it  was,  had 
gone  beyond  the  empirical  observation  that  a  common  convex 
spectacle  lens  and  a  concave  one  of  relatively  large  curvature 
could  be  placed  in  a  tube,  convex  ahead,  at  such  a  distance  apart 
as  to  give  a  clear  enlarged  image  of  distant  objects. 

It  remained  for  Galileo  (1564-1647)  to  grasp  the  general 
principles  involved  and  to  apply  them  to  a  real  instrument  of 
research.  It  was  in  May  1609  that,  on  a  visit  to  Venice,  he  heard 
reports  that  a  Belgian  had  devised  an  instrument  which  made 
distant  objects  seem  near,  and  this  being  quickly  confirmed  by  a 
letter  from  Paris  he  awakened  to  the  importance  of  the  issue  and, 
returning  to  Padua,  is  said  to  have  solved  the  problem  the  very 
night  of  his  arrival. 

Next  day  he  procured  a  plano-convex  and  a  plano-concave 
lens,  fitted  them  to  a  lead  tube  and  found  that  the  combination 
magnified  three  diameters,  an  observation  which  indicates  about 
what  it  was  possible  to  obtain  from  the  stock  of  the  contemporary 
spectacle  maker.  ^     The  relation  between  the  power  and  the  foci 

1  The  statement  by  Galileo  that  he  "fashioned"  these  first  lenses  can 
hardly  be  taken  literally  if  his  very  speedy  construction  is  to  be  credited. 


8 


THE  TELESCOPE 


of  the  lenses  he  evidently  quickly  fathomed  for  his  next  recorded 
trial  reached  about  eight  diameters. 

With  this  instrument  he  proceeded  to  Venice  and  during  a 
month's  stay,  August,  1609,  exhibited  it  to  the  senators  of  the 
repubhc  and  throngs  of  notables,  finally  disclosing  the  secret  of 
its  construction  and  presenting  the  tube  itself  to  the  Doge 
sitting  in  full  council.  This  particular  telescope  was  about 
twenty  inches  long  and  one  and  five  eighths  inches  in  aperture, 
showing  plainly  that  Galileo  had  by  this  time  found,  or  more 


Lodge  "Pioneers  of  Science." 
Fig.  3.— Galileo. 


likely  made,  an  eye  lens  of  short  focus,  about  three  inches,  quite 
probably  using  a  well  polished  convex  lens  of  the  ordinary  sort 
as  objective 

Laden  with  honors  he  returned  to  Padua  and  settled  down  to 
the  hard  work  of  development,  grinding  many  lenses  with  his 
own  hands  and  finally  producing  the  instrument  magnifying 
some  32  times,  with  which  he  began  the  notable  succession  of 
discoveries  that  laid  the  foundation  of  observational  astronomy. 
This  with  another  of  similar  dimensions  is  still  preserved  at  the 


THE  EVOLUTION  OF  THE  TELESCOPE  9 

Galileo  Museum  in  Florence,  and  is  shown  in  the  Frontispiece. 
The  larger  instrument  is  forty-nine  inches  long  and  an  inch  and 
three  quarters  aperture,  the  smaller  about  thirty-seven  inches  long 
and  of  an  inch  and  five-eighths  aperture.  The  tubes  are  of 
paper,  the  glasses  still  remain,  and  these  are  in  fact  the  first  astro- 
nomical telescopes. 

Gahleo  made  in  Padua,  and  after  his  return  to  Florence  in  the 
autumn  of  1610,  many  telescopes  which  found  their  way  over 
Europe,  but  quite  certainly  none  of  power  equalling  or  exceeding 
these. 

In  this  connection  John  Greaves,  later  Savilian  Professor  of  As- 
tronomy at  Oxford,  writing  from  Sienna  in  1639,  says:  "Galileus 
never  made  but  two  good  glasses,  and  those  were  of  old  Venice 
glass."  In  these  best  telescopes,  however,  the  great  Florentine 
had  clearly  accomplished  a  most  workmanlike  feat.  He  had 
brought  the  focus  of  his  eye  lens  down  to  that  usual  in  modern 
opera  glasses,  and  has  pushed  his  power  about  to  the  limit  for 
simple  lenses  thus  combined. 

The  lack  of  clear  and  homogeneous  glass,  the  great  difficulty 
of  forming  true  tools,  want  of  suitable  commercial  abrasives, 
impossibility  of  buying  sheet  metals  or  tubing  (except  lead), 
and  default  of  now  familiar  methods  of  centering  and  testing 
lenses,  made  the  production  of  respectably  good  instruments  a 
task  the  difficulty  of  which  it  is  hard  now  to  appreciate. 

The  services  of  Galileo  to  the  art  were  of  such  profound  impor- 
tance, that  his  form  of  instrument  may  well  bear  his  name,  even 
though  his  eyes  were  not  the  first  that  had  looked  through  it. 
Such,  too,  was  the  judgment  of  his  contemporaries,  and  it  was 
by  the  act  of  his  colleagues  in  the  renowned  Acaddemia  dei 
Lincei,  through  the  learned  Damiscianus,  that  the  name  "Tele- 
scope" was  devised  and  has  been  handed  down  to  us. 

A  serious  fault  of  the  Galilean  telescope  was  its  very  small 
field  of  view  when  of  any  considerable  power.  Galileo's  largest 
instrument  had  a  field  of  but  7'15",  less  than  one  quarter  the 
moon's  diameter.  The  general  reason  is  plain  if  one  follows  the 
rays  through  the  lenses  as  in  Fig.  4  where  A 5  is  the  distant  object, 
0  the  objective,  e  the  eye  lens,  ah  the  real  image  in  the  absence  of  e, 
and  a'h'  the  virtual  magnified  image  due  to  e. 

It  will  be  at  once  seen  that  the  axes  of  the  pencils  of  rays  from 
all  parts  of  the  object,  as  shown  by  the  heavy  lines,  act  as  if  they 
diverged  from  the  optical  center  of  the  objective,  but  diverging 


10 


THE  TELESCOPE 


still  more  by  refraction  through  the  concave  eye  lens  e,  fall  mostly 
outside  the  pupil  of  the  observer's  eye.  In  fact  the  field  is 
approximately  measured  by  the  angle  subtended  by  the  pupil 
from  the  center  of  o. 

To  the  credit  of  the  Galilean  form  may  be  set  down  the  con- 
venient erect  image,  a  sharp,  if  small,  field  somewhat  bettered 
by  a  partial  compensation  of  the  aberrations  of  the  objective  by 
the  concave  eye  lens,   and  good  illumination.     For  a  distant 


Fig.  4. — Diagram  of  Galileo's  Telescope. 

object  the  lenses  were  spaced  at  the  difference  of  their  focal 
lengths,  and  the  magnifying  power  was  the  ratio  of  these,  fo/fe- 

But  the  difficulty  of  obtaining  high  power  with  a  fairly  sizeable 
field  was  ultimately  fatal  and  the  type  now  survives  only  in  the 
form  of  opera  and  field  glasses,  usually  of  2  to  5  power,  and  in  an 
occasional  negative  eye  lens  for  erecting  the  image  in  observatory 
work.  Practically  all  the  modern  instruments  have  achromatic 
objectives  and  commonly  achromatic  oculars. 


Fig.  5. — Diagram  of  Kepler's  Telescope. 

The  necessary  step  forward  was  made  by  Johann  Kepler 
(1571-1630),  the  immortal  discoverer  of  the  laws  of  planetary 
motion.  In  his  Dioptrice  (1611)  he  set  forth  the  astronom- 
ical telescope,  substantially,  save  for  the  changes  brought  by 
achromatism,  as  it  has  been  used  ever  since.  His  arrangement 
was  that  of  Fig.  5  in  which  the  letters  have  the  same  significance 
as  in  Fig.  4. 

There  are  here  three  striking  differences  from  the  Galilean 
form.  There  is  a  real  image  in  the  front  focus  of  the  eye  lens  e, 
the  rays  passing  it  are  refracted  inwards  instead  of  outwards, 
to  the  great  advantage  of  the  field,  and  any  object  placed  in  the 
image  plane  will  be  magnified  together  with  the  image.     The 


THE  EVOLUTION  OF  THE  TELESCOPE  11 

first  two  points  Kepler  fully  realized,  the  third  he  probably  did 
not,  though  it  is  the  basis  of  the  micrometer.  The  lenses  o  and 
e  are  obviously  spaced  at  the  sum  of  their  focal  lengths,  and  as 
before  the  magnifying  power  is  the  ratio  of  these  lengths,  the  visi- 
ble image  being  inverted. 

Kepler,  so  far  as  known,  did  not  actually  use  the  new  telescope, 
that  honor  falling  about  half  a  dozen  years  later,  to  Christopher 
Scheiner,  a  Jesuit  professor  of  mathematics  at  Ingolstadt,  best 
known  as  a  very  early  and  most  persistent,  not  to  say  verbose, 
observer  of  sun  spots.  His  Rosa  Ursina  (1630)  indicates  free 
use  of  Kepler's  telescope  for  some  years  previously,  in  just  what 
size  and  power  is  uncertain.^  Fontana  of  Naples  also  appears 
to  have  been  early  in  the  field. 

But  the  new  instrument  despite  its  much  larger  field  and  far 
greater  possibilities  of  power,  brought  with  it  some  very  serious 
problems.  With  increased  power  came  greatly  aggravated 
trouble  from  spherical  aberration  and  chromatic  aberration  as 
well,  and  the  additive  aberrations  of  the  eye  lens  made  matters 
still  worse.  The  earlier  Keplerian  instruments  were  probably 
rather  bad  if  the  drawings  of  Fontana  from  1629  to  1636 
fairly  represent  them. 

If  one  may  judge  from  the  course  of  developments,  the  first 
great  impulse  to  improvement  came  with  the  publication  of 
Descartes'  (1596-1650)  study  of  dioptrics  in  1637.  Therein 
was  set  forth  much  of  the  theory  of  spherical  aberration  and 
astronomers  promptly  followed  the  clues,  practical  and  impracti- 
cal, thus  disclosed. 

Without  going  into  the  theory  of  aberrations  the  fact  of  im- 
portance to  the  improvement  of  the  early  telescope  is  that  the 
longitudinal  spherical  aberration  of  any  simple  lens  is  directly  pro- 
portional to  its  thickness  due  to  curvature.  Hence,  other  things 
being  equal,  the  longer  the  focus  for  the  same  aperture  the  less  the 
spherical  aberration  both  absolutely  and  relatively  to  the  image. 
Further,  although  Descartes  knew  nothing  of  chromatic  aber- 
ration, and  the  colored  fringe  about  objects  seen  through  the  tele- 
scope must  then  have  seemed  altogether  mysterious,  it,  also,  was 
greatly  relieved  by  lengthening  the  focus. 

^  Scheiner  also  devised  a  crude  parallactic  mount  which  he  used  in  his  solar 
observations,  probably  the  first  European  to  grasp  the  principle  of  the 
equatorial.  It  was  only  near  the  end  of  the  century  that  Roemer  followed  his 
example,  and  both  had  been  anticipated  by  Chinese  instruments  with  sights. 


12 


THE  TELESCOPE 


For  the  chromatic  circle  produced  by  a  simple  lens  of  given 
diameter  has  a  radial  width  substantially  irrespective  of  the  focal 
length.  But  increasing  the  focal  length  increases  in  exact  propor- 
tion the  size  of  the  image,  correspondingly  decreasing  the  relative 
effect  of  the  chromatic  error. 

Descartes  also  suggested  several  designs  of  lenses  which  would 
be  altogether  free  of  spherical  aberration,  formed  with  elliptical 
or  hyperbolic  curvature,  and  for  some  time  fruitless  efforts  were 
made  to  realize  this  in  practice.  It  was  in  fact  to  be  near  a 
century  before  anyone  successfully  figured  non-spherical  surfaces. 
It  was  spherical  quite  as  much  as  chromatic  aberration  that 
drove  astronomers  to  long  telescopes. 

Meanwhile  the  astronomical  telescope  fell  into  better  hands 
than  those  of  Scheiner.  The  first  fully  to  grasp  its  possibilities 
was  William  Gascoigne,  a  gallant  young  gentleman  of  Middleton, 
Yorkshire,  born  about  1620  (some  say  as  early  as  1612)  and  who 
died  fighting  on  the  King's  side  at  Marston  Moor,  July  2,  1644. 
To  him  came  as  early  as  1638  the  inspiration  of  utilizing  the  real 
focus  of  the  objective  for  establishing  a  telescopic  sight. 


I 

.i_ 
a' 


Fig.   6. — Diagram  of  Terrestrial   Ocular. 

This  shortly  took  the  form  of  a  genuine  micrometer  consisting 
of  a  pair  of  parallel  blades  in  the  focus,  moved  in  opposite  direc- 
tions by  a  screw  of  duplex  pitch,  with  a  scale  for  whole  revolutions, 
and  a  head  divided  into  100  parts  for  partial  revolutions.  With 
this  he  observed  much  from  1638  to  1643,  measured  the  diameters 
of  sun,  moon  and  planets  with  a  good  degree  of  precision,  and 
laid  the  foundations  of  modern  micrometry.  He  was  equipped 
by  1639  with  what  was  then  called  a  large  telescope. 

His  untimely  death,  leaving  behind  an  unpublished  treatise 
on  optics,  was  a  grave  loss  to  science,  the  more  since  the  manu- 
script could  not  be  found,  and,  swept  away  by  the  storms  of  war, 
his  brilliant  work  dropped  out  of  sight  for  above  a  score  of  years. 

Meanwhile  De  Rheita  (1597-1660),  a  Capuchin  monk,  and  an 
industrious  and  capable  investigator,  had  been  busy  with  the 


THE  EVOLUTION  OF  THE  TELESCOPE 


13 


telescope,  and  in  1645  published  at  Antwerp  a  somewhat  bizarre 
treatise,  dedicated  to  Jesus  Christ,  and  containing  not  a  little 
practical  information.  De  Rheita  had  early  constructed  bin- 
oculars, probably  quite  independently,  had  lately  been  diligently 
experimenting  with  Descartes'  hyperbolic  lens,  it  is  needless  to 
say  without  much  success,  and  was  meditating  work  on  a  colossal 
scale — a  glass  to  magnify  4,000  times. 

But  his  real  contribution  to  optics  was  the  terrestrial  ocular. 
This  as  he  made  it  is  shown  in  Fig.  6  where  a  5  is  the  image 


Fig.  7. — Johannes  Hevelius. 


formed  by  the  objective  in  front  of  the  eye  lens  r,  s  and  t  two 
equal  lenses  separated  by  their  focal  lengths  and  a'  h'  the  resultant 
reinverted  image.  This  form  remained  in  common  use  until 
improved  by  Dolland  more  than  a  century  later. 

A  somewhat  earlier  form  ascribed  to  Father  Scheiner  had 
merged  the  two  lenses  forming  the  inverting  system  of  Fig. 
6,  into  a  single  lens  used  at  its  conjugate  foci. 

Closely  following  De  Rheita  came  Johannes  Hevelius  (1611- 
1687)  of  Danzig,  one  of  the  really  important  observers  of  the 


14  THE  TELESCOPE 

seventeenth  century.  His  great  treatise  Selenographia  published 
in  1647  gives  us  the  first  systematic  study  of  the  moon,  and  a  brief 
but  illuminating  account  of  the  instruments  of  the  time  and 
their  practical  construction. 

At  this  time  the  Galilean  and  Keplerian  forms  of  telescope  were 
in  concurrent  use  and  Hevelius  gives  directions  for  designing  and 
making  both  of  them.  Apparently  the  current  instruments  were 
not  generally  above  five  or  six  feet  long  and  from  Hevehus'  data 
would  give  not  above  30  diameters  in  the  Galilean  form.  There 
is  mention,  however,  of  tubes  up  to  12  feet  in  length,  and  of  the 
advantage  in  clearness  and  power  of  the  longer  focus  plano- 
convex lens.  Paper  tubes,  evidently  common,  are  condemned, 
also  those  of  sheet  iron  on  account  of  their  weight,  and  wood  was 
to  be  preferred  for  the  longer  tubes. 

Evidently  Hevelius  had  at  this  time  no  notion  of  the  effect  of 
the  plano-convex  form  of  lens  as  such  in  lessening  aberration,  but 
he  mentions  a  curious  form  of  telescope,  actually  due  to  De  Rheita, 
in  which  the  objective  is  double,  apparently  of  two  plano-convex 
lenses,  the  weaker  ahead,  and  used  with  a  concave  eye  lens. 
If  properly  proportioned  such  a  doublet  would  have  less  than  a 
quarter  the  spherical  aberration  of  the  equivalent  double  convex 
lens. 

Hevelius  also  mentions  the  earlier  form  of  reinverting  telescope 
above  referred  to,  and  speaks  rather  highly  of  its  performance. 
To  judge  from  his  numerous  drawings  of  the  moon  made  in  1643 
and  1644,  his  telescopes  were  much  better  than  those  of  Scheiner 
and  Fontana,  but  still  woefully  lacking  in  sharp  definition. 

Nevertheless  the  copper  plates  of  the  Selenographia,  represent- 
ing every  phase  of  the  moon,  placed  the  lunar  details  with  remark- 
able accuracy  and  formed  for  more  than  a  century  the  best  lunar 
atlas  available.  One  acquires  an  abiding  respect  for  the  patience 
and  skill  of  these  old  astronomers  in  seeing  how  much  they  did 
with  means  utterly  inadequate. 

One  may  get  a  fair  idea  of  the  size,  appearance,  and  mounting 
of  telescopes  in  this  early  day  from  Fig.  8,  which  shows  a  somewhat 
advanced  construction  credited  by  Hevelius  to  a  suggestion  in 
Descartes'  Dioptrica.  Appearances  indicate  that  the  tube  was 
somewhere  about  six  feet  long,  approximately  two  inches  in 
aperture,  and  that  it  had  a  draw  tube  for  focussing.  The  offset 
head  of  the  mount  to  allow  observing  near  the  zenith  is  worth 
an  extra  glance. 


THE  EVOLUTION  OF  THE  TELESCOPE 


15 


Incidentally  Hevelius,  with  perhaps  pardonable  pride,  also 
explains  the  "Polemoscope,"  alittleinventionof  his  own,  made,  he 
tells  us,  in  1637.  It  is  nothing  else  than  the  first  periscope, 
constructed  as  shown  in  Fig.  9,  a  tube  c  with  two  right  angled 


Fig.  8. — A  SeEES^i  Century  Astronomer  and  his  Telescope. 


branches,  a  fairly  long  one  e  for  the  objective/,  a  45°  mirror  at  g, 
another  at  a,  and  finally  the  concave  ocular  at  h.  It  was  of 
modest  size,  of  tubes  1^^  inch  in  diameter,  the  longer  tube  being 
22  inches  and  the  upper  branch  8  inches,  a  size  well  suited  for 
trench  or  parapet. 


16  THE  TELESCOPE 

Even  in  these  days  of  his  youth  Hevehus  had  learned  much  of 
practical  optics  as  then  known,  had  devised  and  was  using  very 
rational  methods  of  observing  sun-spots  by  projection  in  a  dark- 
ened room,  and  gives  perhaps  the  first  useful  hints  at  testing 
telescopes  by  such  solar  observations  and  on  the  planets.  He  was 
later  to  do  much  in  the  development  and  mounting  of  long  tele- 
scopes and  in  observation,  although,  while  progressive  in  other 
respects,  he  very  curiously  never  seemed  to  grasp  the  importance 
of  telescopic  sights  and  consistently  refused  to  use  them. 

Telescope  construction  was  now  to  fall  into  more  skillful  hands. 
Shortly  after  1650  Christian  Huygens  (1629-1695),  and  his 
accomplished  brother  Constantine  awakened  to  a  keen  interest 
in  astronomy  and  devised  new  and  excellent  methods  of  forming 
accurate  tools  and  of  grinding  and  polishing  lenses. 


Fig.  9. — The  first  Periscope. 

By  1655  they  had  completed  an  instrument  of  12  feet  focus  with 
which  the  study  of  Saturn  was  begun.  Titan  the  chief  satellite 
discovered,  and  the  ring  recognized.  Pushing  further,  they 
constructed  a  telescope  of  23  feet  focal  length  and  23^^  inches 
aperture,  with  which  four  years  later  Christian  Huygens  finally 
solved  the  mystery  of  Saturn's  ring. 

Evidently  this  glass,  which  bore  a  power  of  100,  was  of  good 
defining  quality,  as  attested  by  a  sketch  of  Mars  late  in  1695 
showing  plainly  Syrtis  Major,  from  observation  of  which  Huy- 
gens determined  the  rotation  period  to  be  about  24  hours. 

The  Huygens  brothers  were  seemingly  the  first  fully  to  grasp 
the  advantage  of  very  long  focus  in  cutting  down  the  aberrations, 
the  aperture  being  kept  moderate.  Their  usual  proportions  were 
about  as  indicated  above,  the  aperture  being  kept  somewhere 
nearly  as  the  square  root  of  the  focus  in  case  of  the  larger  glasses. 


THE  EVOLUTION  OF  THE  TELESCOPE 


17 


In  the  next  two  decades  the  focal  length  of  telescopes  was 
pushed  by  all  hands  to  desperate  extremes.  The  Huygens 
brothers  extended  themselves  to  glasses  up  to  210  feet  focus  and 
built  many  shorter  ones,  a  famous  example  of  which,  of  6  inches 
aperture  and  123  feet  focal  length,  presented  to  the  Royal 
Society,  is  still  in  its  possession.  Auzout  produced  even  longer 
telescopes,  and  Divini  and  Campani,  in  Rome,  of  whom  the  last 
named  made  Cassini's  telescopes  for  the  Observatory  of  Paris, 
were  not  far  behind.  The  English  makers  were  similarly  busy, 
and  Hevelius  in  Danzig  was  keeping  up  the  record. 


Fig.  10. — Christian  Huygens. 


Clearly  these  enormously  long  telescopes  could  not  well  be 
mounted  in  tubes  and  the  users  were  driven  to  aerial  mountings, 
in  which  the  objective  was  at  the  upper  end  of  a  spar  or  girder 
and  the  eye  piece  at  the  lower.  Figure  11  shows  an  actual  con- 
struction by  Hevelius  for  an  objective  of  150  feet  focal  length. 

In  this  case  the  main  support  was  a  T  beam  of  wooden  planks 
well  braced  together.  Additional  stiffness  was  given  by  light 
wooden  diaphragms  at  short  intervals  with  apertures  of  about 
8  inches  next  to  the  objective,  and  gradually  increasing  down- 
wards. The  whole  was  lined  up  by  equalizing  tackle  in  the  verti- 
cal plane,  and  spreaders  with  other  tackle  at  the  joints  of  the  40- 


18 


THE  TELESCOPE 


foot  sections  of  the  main  beam.     The  mast  which  supported 
the  whole  was  nearly  90  feet  high. 

So  unwieldly  and  inconvenient  were  these  long  affairs  that, 
quite  apart  from  their  usual  optical  imperfections,  it  is  little 
wonder  that  they  led  to  no  results  commensurate  with  their  size. 
In  fact  nearly  all  the  productive  work  was  done  with  telescopes 
from  20  to  35  feet  long,  with  apertures  roughly  between  2  and 
3  inches. 


Fig.  11. — Hevelius'  150-foot  Telescope. 


Dominique  Cassini  to  be  sure,  scrutinizing  Saturn  in  1684  with 
objectives  by  Campani,  of  100  and  136  feet  focus  picked  up  the 
satellites  Tethys  and  Dione,  but  he  had  previously  found  lapetus 
with  a  17-foot  glass,  and  Rhea  with  one  of  34  feet.  The  longer 
glasses  above  mentioned  had  aerial  mounts  but  the  smaller 
ones  were  in  tubes  supported  on  a  sort  of  ladder  tripod.  A  20- 
foot  objective,  power  90,  gave  Cassini  the  division  in  Saturn's  ring. 

A  struggle  was  still  being  kept  up  for  the  non-spherical  curves 
urged  by  Descartes.     It  is  quite  evident  that  Huygens  had  a  go 


THE  EVOLUTION  OF  THE  TELESCOPE 


19 


at  them,  and  Hevelius  thought  at  one  time  that  he  had  mastered 
the  hyperboKc  figure,  but  his  pubhshed  drawings  give  no  indica- 
tion that  he  had  reduced  spherical  aberration  to  any  perceptible 
degree.  At  this  time  the  main  thing  was  to  get  good  glass  and 
give  it  true  figure  and  polish,  in  which  Huygens  and  Campani 
excelled,  as  the  work  on  Saturn  witnesses. 

These  were  the  days  of  the  dawn  of  popular  astronomy  and 
many  a  gentleman  was  aroused  to  at  least  a  casual  interest  in 
observing  the  Heavens.  Notes  Pepys  in  his  immortal  Diary: 
"1  find  Reeves  there,  it  being  a  mighty  fine  bright  night,  and  so 
upon  my  leads,  though  very  sleepy,  till  one  in  the  morning,  look- 
ing on  the  moon  and  Jupiter,  with  this  twelve  foot  glass,  and 
another  of  six  foot,  that  he  hath  brought  with  him  to-night, 
and  the  sights  mighty  pleasant,  and  one  of  the  glasses  I  will  buy." 

Little  poor  Pepys  probably  saw,  by  reason  of  his  severe 
astigmatism,  but  astronomy  was 
in  the  air  with  the  impulse  that 
comes  to  every  science  after  a 
period  of  brilliant  discovery.  An 
other  such  stimulus  came  near  the 
end  of  the  eighteenth  century, 
with  the  labors  of  Sir  William 
Herschel. 

Just  at  this  juncture  comes  one 
of  the  interesting  episodes  of  tele- 
scopic history,  the  ineffectual  and 
abandoned  experiments  on  reflect- 
ing instruments. 

In  1663  James  Gregory  (1638- 
1675)  a  famous  Scottish  mathe- 
matician, published  his  Optica 
Promota,  in  which  he  described 
the  rather  elegant  construction 
which  bears  his  name,  a  perforated  parabolic  mirror  with  an 
elliptical  mirror  forward  of  the  focus  returning  an  image  to  the 
ocular  through  the  perforation.  It  was  convenient  in  that  it 
gave  an  erect  image,  and  it  was  sound  theoretically,  and,  as  the 
future  proved,  practically,  but  the  curves  were  quite  too  much 
for  the  contemporary  opticians.  Figure  12  shows  the  diagram- 
matic construction  as  published. 

The  next  year   Gregory   started   Reive,   a  London   optician, 


\/ 


Fig.    12.- 


-Gregory's    Diagram    of 
his  Telescope. 


20  THE  TELESCOPE 

doubtless  the  same  mentioned  by  Pcpys,  on  the  construction  of  a 
6  foot  telescope.  This  rather  ambitious  effort  failed  of  material 
success  through  the  inability  of  Reive  to  give  the  needed  figures 
to  the  mirrors/  and  of  it  nothing  further  appears  until  the  ingen- 
nious  Robert  Hooke  (1635-1703)  executed  in  1674  a  Grego- 
rian, apparently  without  any  notable  results.  There  is  a  well 
defined  tradition  that  Gregory  himself  was  using  one  in  1675,  at 
the  time  of  his  death,  but  the  invention  then  dropped  out  of  sight. 

No  greater  influence  on  the  art  attended  the  next  attempt  at  a 
reflector,  by  Isaac  Newton  (1643-1727).  This  was  an  early  out- 
come of  his  notable  discovery  of  the  dispersion  of  light  by  prisms, 
which  led  him  to  despair  of  improving  refracting  telescopes  and 
turned  his  mind  to  reflectors. 

Unhappily  in  an  experiment  to  determine  whether  refraction 
and  dispersion  were  proportional  he  committed  the  singular 
blunder  of  raising  the  refractive  index  of  a  water-filled  prism  to 
equality  with  glass  by  dissolving  sugar  of  lead  in  it.  Without 
realizing  the  impropriety  of  thus  varying  two  quite  unknown 
quantities  at  once  in  his  crucial  experiment,  he  promptly  jumped 
to  the  conclusion  that  refraction  and  dispersion  varied  in  exact 
proportion  in  all  substances,  so  that  if  two  prisms  or  lenses 
dispersed  light  to  the  same  extent  they  must  also  equally  refract 
it.  It  would  be  interesting  to  know  just  how  the  fact  of  his 
bungling  was  passed  along  to  posterity.  As  a  naive  apolo- 
gist once  remarked,  it  was  not  to  be  found  in  his  "Optics." 
But  Sir  David  Brewster  and  Sir  John  Herschel,  both  staunch 
admirers  of  the  great  philosopher,  state  the  fact  very  positively. 
If  one  may  hazard  a  guess  it  crept  out  at  Cambridge  and  was 
passed  along,  perhaps  to  Sir  William  Herschel,  via  the  unpub- 
lished history  of  research  that  is  rich  in  picturesque  details  of  the 
mare's  nests  of  science.  At  all  events  a  mistake  with  a  great 
name  behind  it  carries  far,  and  the  result  was  to  delay  the 
production  of  the  achromatic  telescope  by  some  three  quarters 
of  a  century. 

Turning  from  refractors  he  presented  to  the  Royal  Society 
just  after  his  election  as  Fellow  in  1672,  the  little  six-inch  model  of 
his  device  which  was  received  with  acclamation  and  then  lay  on 
the  shelf  without  making  the  slightest  impression  on  the  art,  for 
full  half  a  century. 

^  He  attempted  to  polish  them  on  cloth,  which  in  itself  was  sufficient  to 
guarantee  failure. 


THE  EVOLUTION  OF  THE  TELESCOPE 


21 


Newton,  by  dropping  the  notion  of  direct  view  through  the 
tube,  hit  upon  by  far  the  simplest  way  of  getting  the  image  out- 
side it,  by  a  plane  mirror  a  little  inside  focus  and  inclined  at  45°, 
but  injudiciously  abandoned  the  parabolic  mirror  of  his  original 
paper  on  dispersion.  His  invention  therefore  as  actually 
made  public  was  of  the  combination  with  a  spherical  concave 
mirror  of  a  plane  mirror  of  elliptical  form  at  45°,  a  construction 
which  in  later  papers  he  defended  as  fully  adequate.^ 


Fig.  13. — Newton's   Model   of  his  Reflector. 


His  error  in  judgment  doubtless  came  from  lack  of  practical 
astronomical  experience,  for  he  assumed  that  the  whole  real 
trouble  with  existing  telescopes  was  chromatic  aberration,  which 
in  fact  worried  the  observer  little  more  than  the  faults  due  to 
other  causes,   since  the  very  low  luminosity  toward  the  ends 

^  In  Fig  13,  A  is  the  support  of  the  tube  and  focussing  screw,  B  the  main 
mirror,  an  inch  in  diameter,  CD  the  obhque  mirror,  E  the  principal  focus, 
F  the  eye  lens,  and  G  the  member  from  which  the  oblique  mirror  is  carried. 


22  THE  TELESCOPE 

of  the  spectrum  enormously  lessens  the  indistinctness  duo  to 
dispersion. 

As  a  matter  of  fact  the  long  focus  objective  of  small  aperture 
did  very  creditable  work,  and  its  errors  would  not  compare 
unfavorably  with  those  of  a  spherical  concave  mirror  of  the  wide 
aperture  planned  by  Newton.  Had  he  actually  made  one  of 
his  telescopes  of  fair  dimensions  and  power  the  definition  would 
infallibly  have  been  wrecked  by  the  aberrations  due  to  spherical 
figure.^ 

It  is  quite  likely  that  appreciation  of  this,  and  the  grave  doubts 
of  both  Newton  and  Huygens  as  to  obtaining  a  proper  parabolic 
curve  checked  further  developments.     About  the  beginning  of 


Fig.   14. — De  Berce's  sketch  of  Cassegrain's  Telescope. 

the  year  1672  M.  Cassegrain  communicated  to  M.  de  Berc^a 
design  for  a  reflecting  telescope,  which  eventually  found  its  way 
into  the  Philosophical  Transactions  of  May  in  that  year,  after 
previous  publication  in  the  Journal  des  Sgavans.  Figure  14  shows 
de  Berce's  rough  original  sketch.  It  differed  from  Gregory's 
construction  in  that  the  latter's  elliptical  concave  mirror  placed 
outside  the  main  focus,  was  replaced  by  a  convex  mirror  placed 
inside  focus.     The  image  was  therefore  inverted. 

The  inventor  is  referred  to  in  histories  of  science  as  ''Casse- 
grain, a  Frenchman."  He  was  in  fact  Sieur  Guillaume  Cassegrain, 
sculptor  in  the  service  of  Louis  Quatorze,  modeller  and  founder 
of  many  statues.     In  1666  he  was  paid  1200  livres  for  executing 

1  In  fact  a  "four  foot  telescope  of  Mr.  Newton's  invention"  brought 
before  the  Royal  Society  two  weeks  after  his  original  paper,  proved  only 
fair  in  quality,  was  returned  somewhat  improved  at  the  next  meeting, 
and  then  was  referred  to  Mr.  Hooke  to  be  perfected  as  far  as  might  be, 
after  which  nothing  more  was  heard  of  it. 


THE  EVOLUTION  OF  THE  TELESCOPE  23 

a  bust  of  the  King  modelled  by  Bertin,  and  later  made  many 
replicas  from  the  antique  for  the  decoration  of  His  Majesty's 
gardens  at  Versailles.  He  disappeared  from  the  royal  records  in 
1684  and  probably  died  within  a  year  or  two  of  that  date. 

At  the  period  here  concerned  he  apparently,  like  de  Berce,  was 
of  Chartres.  Familiar  with  working  bronzes  and  with  the  art 
of  the  founder,  he  was  a  very  likely  person  to  have  executed' 
specula.  Although  there  is  no  certainty  that  he  actually  made  a 
telescope,  a  contemporary  reference  in  the  Journal  des  Sgavans 
speaks  of  his  invention  as  a  "petite  lunette  d'approche,"  and  one 
does  not  usually  suggest  the  dimensions  of  a  thing  non-existent. 
How  long  he  had  been  working  upon  it  prior  to  the  period  about 
the  beginning  of  1672  when  he  disclosed  the  device  to  de  Berce 
is  unknown. 

Probably  Newton's  invention  was  the  earlier,  but  the  two  were 
independent,  and  it  was  somewhat  ungenerous  of  Newton  to 
criticise  Cassegrain,  as  he  did,  for  using  spherical  mirrors,  on  the 
strength  of  de  Berce's  very  superficial  description,  when  he  him- 
self considered  the  parabolic  needless. 

However,  nothing  further  was  done,  and  the  devices  of  Gregory, 
Newton  and  Cassegrain  went  together  into  the  discard  for  some 
fifty  years. 

These  early  experiments  gave  singularly  little  information 
about  material  for  mirrors  and  methods  of  working  it,  so  little 
that  those  who  followed,  even  up  to  Lord  Rosse,  had  to  work 
the  problems  out  for  themselves.  We  know  from  his  original 
paper  that  Newton  used  bell-metal,  whitened  by  the  addition  of 
arsenic,  following  the  lore  of  the  alchemists. 

These  speculative  worthies  used  to  alloy  copper  with  arsenic, 
thinking  that  by  giving  it  a  whitish  cast  they  had  reached  a  sort 
of  half  way  point  on  the  road  to  silver.  Very  silly  at  first 
thought,  but  before  the  days  of  chemical  analysis,  when  the 
essential  properties  of  the  metals  were  unknown,  the  way  of  the 
scientific  experimenter  was  hard. 

What  the  "steely  matter,  imployed  in  London"  of  which 
Newton  speaks  in  an  early  paper  was,  we  do  not  know — very 
likely  one  of  the  hard  alloys  much  richer  in  tin  than  is  ordinary 
bell-metal.  Nor  do  we  know  to  what  variety  of  speculum  metal 
Huygens  refers  in  his  correspondence  with  Newton. 

As  to  methods  of  working  it  Newton  only  disclosed  his  scheme 
of  pitch-polishing  some  thirty  years  after  this  period,  while  it  is 


24  THE  TELESCOPE 

a  matter  of  previous  record,  that  Huygens  had  been  in  the  habit 
of  poHshing  his  true  tools  on  pitch  from  some  date  unknown. 
Probably  neither  of  them  originated  the  practice.  Opticians 
are  a  peculiarly  secretive  folk  and  shop  methods  are  likely  to  be 
kept  for  a  long  time  before  they  leak  out  or  are  rediscovered. 
Modern  speculum  metal  is  substantially  a  definite  compound 
of  four  atoms  copper  and  one  tin  (SnCu4),  practically  68  per 
cent  copper  and  32  per  cent  tin,  and  is  now,  as  it  was  in  all 
previous  modifications,  a  peculiarly  mean  material  to  cast  and 
work.     Thus  exit  the  reflector. 

The  long  telescope  continued  to  grow  longer  with  only  slow 
improvement  in  quality,  but  the  next 
decade  was  marked  by  the  intro- 
duction of  Huygens'  eye-piece,  an  im- 
mense improvement  over  the  single 
lens  which  had  gone  before,  and  with 
slight  modifications  in  use  today. 

This  is  shown  in  section  in  Fig.  15. 
It    consists   of   a   field  lens  A,  plano- 
convex, and  an  eye  lens  B  of  one-third 
Fig.  15.-Diagram  of  Huy-     ^^le  focal  length,  the  two  being  placed 

gen  s  li,ye-piece.  .  . 

at  the  difference  of  their  focal  lengths 
apart  with  (in  later  days)  a  stop  half  way  between  them.  The 
eye  piece  is  pushed  inside  the  main  focus  until  the  ra^^s  which  fall 
on  the  field  lens  focus  through  the  eye  lens. 

The  great  gain  from  Huygens'  view-point  was  a  very  much 
enlarged  clear  field — about  a  four-fold  increase — and  in  fact  the 
combination  is  substantially  achromatic,  particularly  important 
now  when  high  power  oculars  are  needed. 

Still  larger  progress  was  made  in  giving  the  objective  a  better 
form  with  respect  to  spherical  aberration,  the  "  crossed  "  lens  being 
rather  generally  adopted.  This  form  is  double  convex,  and  if  of 
ordinary  glass,  with  the  rear  radius  six  times  the  front  radius,  and 
gives  even  better  results  than  a  plano-convex  in  its  best  position- 
plane  side  to  the  rear.  Objectives  were  rated  on  focal  length  for 
the  green  rays,  that  is,  the  bright  central  part  of  the  spectrum,  the 
violet  rays  of  course  falling  short  and  the  red  running  beyond. 

To  give  customary  dimensions,  a  telescope  of  3  inches  aperture, 
with  magnifying  power  of  100,  would  be  of  about  30  feet  focus 
with  the  violet  nearly  6  inches  short  and  the  red  a  similar  amount 
long.     It  is  vast  credit  to  the  early  observers  that  with  such 


THE  EVOLUTION  OF  THE  TELESCOPE 


25 


slender  means  they  did  so  much.  But  in  fact  the  long  telescope 
had  reached  a  mechanical  impasse,  so  that  the  last  quarter  of  the 
seventeenth  century  and  the  first  quarter  of  the  next  were  marked 
chiefly  by  the  development  of  astronomy  of  position  with  instru- 
ments of  modest  dimensions. 

In  due  time  the  new  order  came  and  with  astounding  sudden- 
ness.    Just   at   the   end   of    1722   James   Bradley    (1692-1762) 


FiQ.  16.— The  First  Ref.ector.     John  Hadley,  1722. 

measured  the  diameter  of  Venus  with  an  objective  of  212  ft. 
3  in.  focal  length;  about  three  months  later  John  Hadley  (1682- 
1744)  presented  to  the  Royal  Society  the  first  reflecting  tele- 
scope worthy  the  name,  and  the  old  order  practically  ended. 

John  Hadley  should  in  fact  be  regarded  as  the  real  inventor  of 
the  reflector  in  quite  the  same  sense  that  Mr.  Edison  has  been 
held,  de  jure  and  de  facto,  the  inventor  of  the  incandescent  elec- 


26  THE  TELESCOPE 

trie  lamp.  Actually  Hadley's  case  is  the  stronger  of  the  two, 
for  the  only  things  which  could  have  been  cited  against  him  were 
abandoned  experiments  fifty  years  old.  MoraQArei'  he  took  suc- 
cessfully the  essential  step  at  which  Gregory  and  Newton  had 
stumbled  or  turned  back — parabolizing  his  speculum. 

The  instrument  he  presented  was  of  approximately  6  inches 
aperture  and  62%  inches  focal  length,  which  he  had  made 
and  tested  some  three  years  previously;  on  a  substantial  altazi- 
muth mount  with  slow  motions.  He  used  the  Newtonian 
oblique  mirror  and  the  instrument  was  provided  with  both 
convex  and  concave  eye  lenses,  with  magnifications  up  to 
about  230. 

The  whole  arrangement  is  shown  in  Fig.  16  which  is  for  the  most 
part  self  explanatory.  It  is  worth  noting  that  the  speculum 
is  positioned  in  the  wooden  tube  by  pressing  it  forward  against 
three  equidistant  studs  by  three  corresponding  screws  at  the  rear, 
that  a  slider  moved  by  a  traversing  screw  in  a  wide  groove 
carries  the  small  mirror  and  the  ocular,  that  there  is  a  convenient 
door  for  access  to  the  mirror^  and  also  a  suitable  finder.  The 
motion  in  altitude  is  obtained  by  a  key  winding  its  cord  against 
gravity.  That  in  azimuth  is  by  a  roller  support  along  a  horizon- 
tal runway  carried  by  an  upright,  and  is  obtained  by  the  key 
with  a  cord  pull  off  in  one  direction,  and  in  the  other,  by  springs 
within  the  main  upright,  turning  a  post  of  which  the  head  carries 
cheek  pieces  on  which  rest  the  trunnions  of  the  tube. 

A  few  months  later  this  telescope  was  carefully  tested,  by 
Bradley  and  the  Rev.  J.  Pound,  against  the  Huygens  objective 
of  123  feet  focus  possessed  by  the  Royal  Society,  and  with 
altogether  satisfactory  results.  Hadley's  reflector  would  show 
everything  which  could  be  seen  by  the  long  instrument,  bearing  as 
much  power  and  with  equal  definition,  though  somewhat  lessened 
light.  In  particular  they  saw  all  five  satellites  of  Saturn,  Cas- 
sini's  division,  which  the  inventor  himself  had  seen  the  previous 
year  even  in  the  northern  edge  of  the  ring  beyond  the  planet,  and 
the  shadow  of  the  ring  upon  the  ball. 

The  casting  of  the  large  speculum  was  far  from  perfect,  with 
many  spots  that  failed  to  take  polish,  but  the  figure  must  have 
been  rather  good.  A  spherical  mirror  of  these  dimensions  would 
give  an  aberration  blur  something  like  twenty  times  the  width  of 
Cassini's  division,  and  the  chance  of  seeing  all  five  satellites 
with  it  would  be  negligibly  small. 


THE  EVOLUTION  OF  THE  TELESCOPE  27 

Further,  Hadley  presently  disclosed  to  others  not  only  the 
method  he  used  in  polishing  and  parabolizing  specula,  but  his 
method  of  testing  for  true  figure  by  the  aberrations  disclosed  as 
he  worked  the  figure  away  from  the  sphere — a  scheme  frequently 
used  even  to  this  day. 

The  effect  of  Hadley's  work  was  profound.  Under  his  guidance 
others  began  to  produce  well  figured  mirrors,  in  particular 
Molyneux  and  Hawksbee;  reflecting  telescopes  became  fairly 
common;  and  in  the  beginning  of  the  next  decade  James  Short, 
(1710-1768),  possessed  of  craftsmanship  that  approached  wiz- 
ardry, not  only  fully  mastered  the  art  of  figuring  the  para- 
boloid, but  at  once  took  up  the  Gregorian  construction  with  its. 
ellipsoidal  small  mirror,  with  much  success. 

His  specula  were  of  great  relative  aperature,  F/4  to  F/6,  and 
from  the  excellent  quality  of  his  metal  some  of  them  have  retained 
their  fine  polish  and  definition  after  more  than  a  century.  He  is 
said  to  have  gone  even  up  to  12  inches  in  diameter.  His  exact 
methods  of  working  died  with  him.  Even  his  tools  he  ordered 
to  be  destroyed  before  his  death. 

The  Cassegrain  reflector,  properly  having  a  parabolic  large 
mirror  and  a  hyperbolic  small  one,  seems  very  rarely  to  have  been 
made  in  the  eighteenth  century,  though  one  certainly  came  into 
the  hands  of  Ramsden  (1735-1800). 

Few  refractors  for  astronomical  use  were  made  after  the  advent 
of  the  reflector,  which  was,  and  is,  however,  badly  suited  for  the 
purposes  of  a  portable  spy-glass,  owing  to  trouble  from  stray 
light.  The  refractor  therefore  permanently  held  its  own  in  this 
function,  despite  its  length  and  uncorrected  aberrations. 

Relief  was  near  at  hand,  for  hardly  had  Short  started  on  his 
notable  career  when  Chester  Moor  Hall,  Esq.  (1704-1771)  a 
gentleman  of  Essex,  designed  and  caused  to  be  constructed  the  first 
achromatic  telescope,  with  an  objective  of  crown  and  flint  glass. 
He  is  stated  to  have  been  studying  the  problem  for  several  years, 
led  to  it  by  the  erroneous  belief  (shared  by  Gregory  long  before) 
that  the  human  eye  was  an  example  of  an  achromatic  instrument. 

Be  this  as  it  may  Hall  had  his  telescopes  made  by  George  Bast 
of  London  at  least  as  early  as  1733,  and  according  to  the  best 
available  evidence  several  instruments  were  produced,  one  of  them 
of  above  2  inches  aperture  on  a  focal  length  of  about  20  inches 
(F/8)  and  further,  subsequently  such  instruments  were  made  and 
sold  by  Bast  and  other  opticians. 


28 


THE  TELESCOPE 


These  facts  are  clear  and  yet,  with  knowledge  of  them  among 
London  workmen  as  well  as  among  Hall's  friends,  the  invention 
made  no  impression,  until  it  was  again  brought  to  light,  and 
patented,  by  the  celebrated  John  Dolland  (1706-1761)  in  the 
year  1758. 

Physical  considerations  give  a  clue  to  this  singular  neglect. 
The  only  glasses  differing  materially  in  dispersion  available  in 


Fig.  17.- 


Lodge  "Pioneers  of  Science.' 
-John    Dolland. 


Hall's  day  were  the  ordinary  crown,  and  such  flint  as  was  in  use 
in  the  glass  cutting  trade, — what  we  would  now  know  as  a  light 
flint,  and  far  from  homogeneous  at  that. 

Out  of  such  material  it  was  practically  very  hard  (as  the 
Dollands  quickly  found)  to  make  a  double  objective  decently  free 
from  spherical  aberration,  especially  for  one  working,  as  Hall  quite 


THE  EVOLUTION  OF  THE  TELESCOPE  29 

assuredly  did,  by  rule  of  thumb.  With  the  additional  handicap 
of  flint  full  of  faults  it  is  altogether  likely  that  these  first  achro- 
matics,  while  embodying  the  correct  principles,  were  not  good 
enough  to  make  effective  headway  against  the  cheaper  and 
simpler  spy-glass  of  the  time. 

Dolland,  although  in  1753  he  strongly  supported  Newton's 
error  in  a  Royal  Society  paper  against  Euler's  belief  in  achro- 
matism, shifted  his  view  a  couple  of  years  later  and  after  a 
considerable  period  of  skilful  and  well  ordered  experimenting  pub- 
lished his  discovery  of  achromatism  early  in  1758,  for  which  a 
patent  was  granted  him  April  19,  while  in  the  same  year  the  Royal 
Society  honored  him  with  the  Copley  medal.  From  that  time 
until  his  death,  late  in  1761,  he  and  his  son  Peter  Dolland  (1730- 
1820)  were  actively  producing  achromatic  glasses. 

The  Dollands  were  admirable  craftsmen  and  their  early  product 
was  probably  considerably  better  than  were  Hall's  objectives  but 
they  felt  the  lack  of  suitable  flint  and  soon  after  John  DoUand's 
death,  about  1765,  the  son  sought  relief  in  the  triple  objective  of 
which  an  early  example  is  shown  in  Fig.  18,  and 
which,  with  some  modifications,  was  his  stan- 
dard form  for  many  years. 

Other  opticians  began  to  make  achromatics, 
and,  Peter  Dolland  having  threatened  action 
for  infringement,  a  petition  was  brought  by  35  ~ — ' 
opticians  of  London  in  1764  for  the  annulment 
of  John  Dolland's  patent,  alleging  that  he  was 
not  the  original  inventor  but  had  knowledge 
of  Chester  Moor  Hall's  prior  work.     In  the 

list  was  George  Bast,  who  in  fact  did  make       fig.    is. Peter 

Hall's    objectives    twenty    five    years    before    Dolland's  Triple  Ob- 

IGCtlVG 

Dolland,  and  also  one  Robert  Rew  of  Cold- 
bath  Fields,  who  claimed  in  1755  to  have  informed  Dolland  of 
the  construction  of  Hall's  objective. 

This  was  just  the  time  when  Dolland  came  to  the  right  about 
face  on  achromatism,  and  it  may  well  be  that  from  Rew  or 
elsewhere  he  may  have  learned  that  a  duplex  achromatic  lens 
had  really  been  produced.  But  his  Royal  Society  paper  shows 
that  his  result  came  from  honest  investigations,  and  at  worst  he 
is  in  about  the  position  of  Galileo  a  century  and  a  half  before. 

The  petition  apparently  brought  no  action,  perhaps  because 
Peter  Dolland  next  year  sued  Champneys,  one  of  the  signers,  and 


30  THE  TELESCOPE 

obtained  judgment.  It  was  in  this  case  that  the  judge  (Lord 
Camden)  dehvered  the  oft  quoted  dictum:  "It  was  not  the 
person  who  locked  up  his  invention  in  his  scrutoire  that  ought  to 
profit  by  a  patent  for  such  invention,  but  he  who  brought  it 
forth  for  the  benefit  of  the  pubhc.^" 

This  was  sound  equity  enough,  assuming  the  facts  to  be  as 
stated,  but  while  Hall  did  not  publish  the  invention  admittedly 
made  by  him,  it  had  certainly  become  known  to  many.  Chester 
Moor  Hall  was  a  substantial  and  respected  lawyer,  a  bencher  of 
the  Inner  Temple,  and  one  is  inclined  to  think  that  his  alleged 
concealment  was  purely  constructive,  in  his  failing  to  contest 
Dolland's  claim. 

Had  he  appeared  at  the  trial  with  his  fighting  blood  up,  there  is 
every  reason  to  believe  that  he  could  have  established  a  perfectly 
good  case  of  public  use  quite  aside  from  his  proof  of  technical 
priority.  However,  having  clearly  lost  his  own  claims  through 
laches,  he  not  improbably  was  quite  content  to  let  the  trades- 
men fight  it  out  among  themselves.  Hall's  telescopes  were  in 
fact  known  to  be  in  existence  as  late  as  1827. 

As  the  eighteenth  century  drew  toward  its  ending  the  reflecting 
telescope,  chiefly  in  the  Gregorian  form,  held  the  field  in  astrono- 
mical work,  the  old  refractor  of  many  draw  tubes  was  the  spy- 
glass of  popular  use,  and  the  newly  introduced  achromatic  was 
the  instrument  of  "the  exclusive  trade."  No  glass  of  suitable 
quality  for  well  corrected  objectives  had  been  produced,  and  that 
available  was  not  to  be  had  in  discs  large  enough  for  gerious 
work.    A  3-inch  objective  was  reckoned  rather  large. 

^  Commonly,  but  it  appears  erroneously,  ascribed  to  Lord  Mansfield. 


CHAPTER  II 

THE  MODERN  TELESCOPE 

The  chief  link  between  the  old  and  the  new,  in  instrumental 
as  well  as  observational  astronomy,  was  Sir_William  Herschel 
(1738-1822).  In  the  first  place  he  carried  the  figuring  of  his 
mirrors  to  a  point  not  approached  by  his  predecessors,  and  second, 
he  taught  by  example  the  immense  value  of  aperture  in  definition 
and  grasp  of  light.  His  life  has  never  been  adequately  written, 
but  Miss  Gierke's  '^The  Herschels  and  Modern  Astronomy ''  is 
extremely  well  worth  the  reading  as  a  record  of  achievement  that 
knew  not  the  impossible. 


Miss  Gierke's  Herschel  &  Modern  Astronomy  (Macmillan). 
Fig.  19.— Sir  William  Herschel. 


He  was  the  son  of  a  capable  band-master  of  Hanover,  brought 
up  as  a  musician,  in  a  family  of  exceptional  musical  abilities, 
and  in  1757  jumped  his  military  responsibilities  and  emigrated  to 
England,  to  the  world's  great  gain.  For  nearly  a  decade  he 
struggled  upward  in  his  art,  taking  meanwhile  every  opportunity 
for  self  education,  not  only  in  the  theory  of  music  but  in  mathe- 
matics and  the  languages,  and  in  1767  we  find  him  settled  in 
fashionable  Bath,  oboist  in  a  famous  orchestra,  and  organist  of 
the  Octagon  Chapel.     His  abilities  brought  him  many  pupils, 

31 


32  THE  TELESCOPE 

and  ultimately  he  became  director  of  the  orchestra  in  which  he 
had  played,  and  the  musical  dictator  of  the  famous  old  resort. 

In  1772  came  his  inspiration  in  the  loan  of  a  2-foot  Gregorian 
reflector,  and  a  little  casual  star-gazing  with  it.  It  was  the 
opening  of  the  kingdom  of  the  skies,  and  he  sought  to  purchase  a 
telescope  of  his  own  in  London,  only  to  find  the  price  too  great  for 
his  means.  (Even  a  2-foot,  of  43^  inches  aperture,  by  Short 
was  listed  at  five-and-thirty  guineas.)  Then  after  some  futile 
attempts  at  making  a  plain  refractor  he  settled  down  to  hard 
work  at  casting  and  polishing  specula. 

Although  possessed  of  great  mechanical  abilities  the  difficult 
technique  of  the  new  art  long  baffled  him,  and  he  cast  and  worked 
some  200  small  discs  in  the  production  of  his  first  successful 
telescopes,  to  say  nothing  of  a  still  greater  number  in  larger  sizes 
in  his  immediately  subsequent  career. 

As  time  went  on  he  scored  a  larger  proportion  of  successes, 
but  at  the  start  good  figure  seems  to  have  been  largely  fortui- 
tous. Inside  of  a  couple  of  years,  however,  he  had  mastered 
something  of  the  art  and  turned  out  a  5-foot  instrument  which 
seems  to  have  been  of  excellent  quality,  followed  later  by  a  7-foot 
(aperture  63^^  inches)  even  better,  and  then  by  others  still  bigger. 

The  best  of  Herschel's  specula  must  have  been  of  exquisite 
figure.  His  7-foot  was  tested  at  Greenwich  against  one  of  Short's 
of  93^^  inches  aperture  much  to  the  latter's  disadvantage.  His 
discovery  with  the  7-foot,  of  the  "Georgium  Sidus"  (Uranus)  in 
1781  won  him  immediate  fame  and  recognition,  beside  spurring 
him  to  greater  efforts,  especially  in  the  direction  of  larger  aper- 
tures, of  which  he  had  fully  grasped  the  importance. 

In  1782  he  successfully  completed  a  12-inch  speculum  of  20  feet 
focus,  followed  in  1788  by  an  18-inch  of  the  same  length.  The 
previous  year  he  first  arranged  his  reflector  as  a  ''front  view" 
telescope — the  so-called  Herschelian.  Up  to  this  time  he,  except 
for  a  few  Gregorians,  had  used  Newton's  oblique  mirror. 

The  heavy  loss  of  light  (around  40  per  cent)  in  the  second 
reflection  moved  him  to  tilt  the  main  mirror  so  as  to  throw  the 
focal  point  to  the  edge  of  the  aperture  where  one  could  look  down- 
ward upon  the  image  through  the  ocular  as  shown  in  Fig.  20. 
Here  SS  is  the  great  speculum,  0  the  ocular  and  i  the  image 
formed  near  the  rim  of  the  tube.  In  itself  the  tilting  would 
seriously  impair  the  definition,  but  Herschel  wisely  built  his 
telescopes  of  moderate  relative  aperture  (F/10  to  F/20),  so  that 


THE  MODERN   TELESCOPE  33 

this  difficulty  was  considerably  lessened,  while  the  saving  of 
light,  amounting  to  nearly  a  stellar  magnitude,  was  important. 

Meanwhile  he  was  hard  at  work  on  his  greatest  mirror,  of 
48  inches  clear  aperture  and  40  feet  focal  length,  the  father  of  the 
great  line  of  modern  telescopes.  It  was  finished  in  the  summer  of 
1789.  The  speculum  was  493-^  inches  in  over-all  diameter,  33--^ 
inches  thick  and  weighed  as  cast  2118  lbs.  The  completion  of 
this  instrument,  which  would  rank  as  large  even  today,  was  made 
notable  by  the  immediate  discovery  of  two  new  satellites  of 
Saturn,  Enceladus  and  Mimas. 

It  also  proved  of  very  great  value  in  sweeping  for  nebulae, 
but  its  usefulness  seems  to  have  been  much  limited  by  the  flexure 
of  the  mirror  under  its  great  weight,  and  by  its  rapid  tarnishing. 
It  required  repolishing,  which  meant  refiguring,  at  least  every  two 
years,  a  prodigious  task.^ 


Fig.  20. — Herschel's  Front  View  Telescope. 

It  was  used  as  a  front  view  instrument  and  was  arranged  as 
shown  in  Fig.  21.  Obviously  the  front  view  form  has  against  it 
the  mechanical  difficulty  of  supporting  the  observer  up  to  quite 
the  full  focal  length  of  the  instrument  in  air,  a  difficulty  vastly 
increased  were  the  mount  an  equatorial  one,  so  that  for  the  great 
modern  reflectors  the  Cassegrain  form,  looked  into  axially  upward, 
and  in  length  only  a  third  or  a  quarter  of  the  principal  focus,  is 
almost  universal. 

As  soon  as  the  excellent  results  obtained  by  Herschel  became 
generally  known,  a  large  demand  arose  for  his  telescopes,  which 
he  filled  in  so  far  as  he  could  spare  the  time  from  his  regular 

1  This  was  probably  due  not  only  to  unfavorable  climate,  but  to  the  fact 
that  Herschel,  with  all  his  ingenuity,  does  not  appear  to  have  mastered 
the  casting  difficulty,  and  was  constrained  to  make  his  big  speculum  of  Cu 
75  per  cent,  Sn  25  per  cent,  a  composition  working  rather  easily  and  taking 
beautiful,  but  far  from  permanent,  polish.  He  never  seems  to  have  used 
practically  the  SnCu4  formula,  devised  empirically  by  Mudge  (Phil.  Trans. 
67,  298),  and  in  quite  general  use  thereafter  up  to  the  present  time. 


34  THE  TELESCOPE 

work,  and  not  the  least  of  his  services  to  science  was  the  distri- 
bution of  telescopes  of  high  quality  and  consequent  strong  stimu- 
lus to  general  interest  in  astronomy. 

Two  of  his  instruments,  of  4-  and  7-feet  focus  respectively,  fell 
into  the  worthy  hands  of  Schroter  at  Lilienthal  and  did  sterling 
service  in  making  his  great  systematic  study  of  the  lunar  surface. 
At  the  start  even  Herschel's  7-foot  telescope  brought  200 -guineas, 
and  the  funds  thus  won  he  promptly  turned  to  research. 


Miss  Gierke's  Herschel  &  Modern  Astronomy  (Macmillan). 
Fig.  21. — Herschel's  Forty-foot  Telescope. 

We  sometimes  think  of  the  late  eighteenth  century  as  a  time 
of  license  unbounded  and  the  higher  life  contemned,  but  Herschel 
wakened  a  general  interest  in  unapplied  science  that  has  hardly 
since  been  equalled  and  never  surpassed.  Try  to  picture  social 
and  official  Washington  rushing  to  do  honor  to  some  astronomer 
who  by  luck  had  found  the  trans-Neptunian  planet;  the  diplo- 
matic corps  crowding  his  doors,  and  his  very  way  to  the  Naval 
Observatory  blocked  by  the  limousines  of  the  curious  and  admir- 
ing, and  some  idea  may  be  gained  of  what  really  happened  to  the 
unassuming  music  master  from  Bath  who  suddenly  found  himself 
famous. 

Great  as  were  the  advances  made  by  Herschel  the  reflector 


THE  MODERN  TELESCOPE  35 

was  destined  to  fall  into  disuse  for  many  years.  The  fact  was 
that  the  specula  had  to  be  refigured,  as  in  the  case  of  the  great  40- 
foot  telescope,  quite  too  often  to  meet  the  requirements  of  the 
ordinary  user,  professional  or  amateur.  Only  those  capable  of 
doing  their  own  figuring  could  keep  their  instruments  con- 
veniently in  service. 

Sir  W.  Herschel  always  had  relays  of  specula  at  hand  for  his 
smaller  instruments,  and  when  his  distinguished  son,  Sir  John 
F.  W.  Herschel,  went  on  his  famous  observing  expedition  to  the 
Cape  of  Good  Hope  in  1834-38  he  took  along  his  polishing 
machine  and  three  specula  for  his  20-foot  telescope.  And  he 
needed  them  indeed,  for  a  surface  would  sometimes  go  bad 
even  in  a  week,  and  regularly  became  quite  useless  in  2  or  3 
months. 

Makers  who  used  the  harder  speculum  metal,  very  brittle  and 
scarcely  to  be  touched  by  a  file,  fared  better,  and  some  small 
mirrors,  well  cared  for,  have  held  serviceable  polish  for  many 
years.  Many  of  these  instruments  of  Herschel's  time,  too,  were 
of  very  admirable  performance. 

Some  of  Herschel's  own  7-foot  telescopes  give  evidence  of 
exquisite  figure  and  he  not  only  commonly  used  magnifying  powers 
up  to  some  80  per  inch  of  aperture,  a  good  stiff  figure  for  a 
telescope  old  or  new,  but  went  above  2,000,  even  nearly  to  6,000 
on  one  of  his  63-^-inch  mirrors  without  losing  the  roundness  of  the 
star  image.  "  Empty  magnification"  of  course,  gaining  no  detail 
whatever,  but  evidence  of  good  workmanship. 

Many  years  later  the  Rev.  W.  R.  Dawes,  the  famous  English 
observer,  had  a  5-inch  Gregorian,  commonly  referred  to  as  ''The 
Jewel,"  on  which  he  used  430  diameters,  and  pushed  to  2,000  on 
Polaris  without  distortion  of  the  disc.  Comparing  it  with  a  5-foot 
(approximately  4-inch  aperture)  refractor,  he  reports  the  Gre- 
gorian somewhat  inferior  in  illuminating  power;  "But  in 
sharpness  of  definition,  smallness  of  discs  of  stars,  and  hardness 
of  outline  of  planets  it  is  superior."  All  of  which  shows  that 
while  methods  and  material  may  have  improved,  the  elders  did 
not  in  the  least  lack  skill. 

The  next  step  forward,  and  a  momentous  one,  was  to  be  taken 
in  the  achromatic  refractor.  Its  general  principles  were  under- 
stood, but  clear  and  homogeneous  glass,  particularly  flint  glass, 
was  not  to  be  had  in  pieces  of  any  size.  "Optical  glass,"  as  we 
understand  the  term,  was  unknown. 


36  THE  TELESCOPE 

It  is  a  curious  and  dramatic  fact  that  to  a  single  man  was  due 
not  only  the  origin  of  the  art  but  the  optical  glass  industry  of  the 
world.  If  the  capacity  for  taking  infinite  pains  be  genius,  then 
the  term  rightfully  belongs  to  Pierre  Louis  Guinand.  He  was  a 
Swiss  artisan  living  in  the  Canton  of  Neuchatel  near  Chaux-de- 
Fonds,  maker  of  bells  for  repeaters,  and  becoming  interested  in 
constructing  telescopes  imported  some  flint  glass  from  England 
and  found  it  bad. 

He  thereupon  undertook  the  task  of  making  better,  and  from 
1784  kept  steadily  at  his  experiments,  failure  only  spurring  him 
on  to  redoubled  efforts.  All  he  could  earn  at  his  trade  went  into 
his  furnaces,  until  gradually  he  won  success,  and  his  glass  began 
to  be  heard  of;  for  by  1799  he  was  producing  flawless  discs  of 
flint  as  much  as  6  inches  in  diameter. 

What  is  more,  to  Guinand  is  probably  due  the  production  of 
the  denser,  more  highly  refractive  flints,  especially  valuable  for 
achromatic  telescopes.  The  making  of  optical  glass  has  always 
been  an  art  rather  than  a  science.  It  is  one  thing  to  know  the 
exact  composition  of  a  glass  and  quite  another  to  know  in  what 
order  and  proportion  the  ingredients  went  into  the  furnace,  to 
what  temperature  they  were  carried,  and  for  how  long,  and  just 
how  the  fused  mass  must  be  treated  to  free  the  products  from 
bubbles  and  striae. 

Even  today,  though  much  has  been  learned  by  scientific  investi- 
gation in  the  past  few  years,  it  is  far  from  easy  to  produce  two 
consecutive  meltings  near  enough  in  refractive  power  to  be 
treated  as  optically  identical,  or  to  produce  large  discs  optically 
homogeneous.  What  Guinand  won  by  sheer  experience  was 
invaluable.  He  was  persuaded  in  1805  to  move  to  Munich  and 
eventually  to  join  forces  with  Fraunhofer,  an  association  which 
made  both  the  German  optical  glass  industry  and  the  modern 
refra^ctor. 

He  returned  to  Switzerland  in  1814  and  continued  to  produce 
perfect  discs  of  larger  and  larger  dimensions.  One  set  of  12  inches 
worked  up  by  Cauchoix  in  Paris  furnished  what  was  for  some 
years  the  world's  largest  refractor. 

Guinaud  died  in  1824,  but  his  son  Henry,  moving  to  Paris, 
brought  his  treasure  of  practical  knowledge  to  the  glass  works 
there,  where  it  has  been  handed  down,  in  effect  from  father  to 
son,  gaining  steadily  by  accretion,  through  successive  firms  to 
the  present  one  of  Parra-Mantois. 


THE  MODERN  TELESCOPE 


37 


Bontemps,  one  of  the  early  pupils  of  Henry  Guinand,  emigrated 
to  England  at  the  Revolution  of  1848  and  brought  the  art  to  the 
famous  firm  of  Chance  in  Birmingham.  Most  of  its  early  secrets 
have  long  been  open,  but  the  minute  teachings  of  experience  are 
a  tremendously  valuable  asset  even  now. 

To  Fraunhofer,  the  greatest  master  of  applied  optics  in  the 
nineteenth  century,  is  due  the  astronomical  telescope  in  sub- 


X 


Fig.  22. — Dr.  Joseph  von  Fraunhofer,  the  Father  of  Astrophysics. 


stantially  its  present  form.  Not  only  did  he  become  under 
Guinand's  instruction  extraordinarily  skillful  in  glass  making 
but  he  practically  devised  the  art  of  working  it  with  mathematical 
precision  on  an  automatic  machine,  and  the  science  of  correctly 
designing  achromatic  objectives. 

The  form  which  he  originated  (Fig.  23)  was  the  first  in  which 
the  aberrations  were  treated  with  adequate  completeness,  and, 
particularly   for   small   instruments,    is   unexcelled   even   now. 


38 


THE  TELESCOPE 


Fig.  23. 


The  curvatures  here  shown  are  extreme,  the  better  to  show  their 
relations.  The  front  radius  of  the  crown  is  about  23^  times 
longer  than  the  rear  radius,  the  front  of  the  flint  is  slightly  flatter 
than  the  back  of  the  crown,  and  the  rear  of  the  flint  is  only 
slightly  convex. 

Fraunhofer's  workmanship  was  of  the  utmost  exactness  and  it 
is  not  putting  the  case  too  strongly  to  say  that  a  first  class  example 
of  the   master's   craft,   in   good   condition,  would 
compare   well  in  color-correction,   definition,    and 
field,  with  the  best  modern  instruments. 

The  work  done  by  the  elder  Struve  at  Dorpat 
with  Fraunhofer's  first  large  telescope  (9.6  inches 
aperture  and  170  inches  focal  length)  tells  the 
story  of  its  quality,  and  the  Konigsberg  helio- 
meter,  the  first  of  its  class,  likewise,  while  even 
today  some  of  his  smaller  instruments  are  still  doing 
good  service. 

It  was  he  who  put  in  practice  the  now  general 
convention  of  a  relative  aperture  of  about  F/15, 
and  standardized  the  terrestrial  eye-piece  into  the  design  quite 
widely  used  today.  The  improvements  since  his  time  have  been 
relatively  slight,  due  mainly  to  the  recent  production  of  varieties 
of  optical  glass  unknown  a  century  ago.  Fraunhofer  was  born 
in  Straubing,  Bavaria,  March  6,  1787.  Self-educated  like 
Herschel,  he  attained  to  an  extraordinary  combination  of 
theoretical  and  practical  knowledge  that  went  far  in  laying  the 
foundations  of  astrophysics. 

The  first  mapping  of  the  solar  spectrum,  the  invention  of  the 
diffraction  grating  and  its  application  to  determining  the  wave 
length  of  light,  the  first  exact  investigation  of  the  refraction  and 
dispersion  of  glass  and  other  substances,  the  invention  of  the 
objective  prism,  and  its  use  in  studying  the  spectra  of  stars  and 
planets,  the  recognition  of  the  correspondence  of  the  sodium  lines 
to  the  D  lines  in  the  sun,  and  the  earliest  suggestion  of  the 
diffraction  theory  of  resolution  later  worked  out  by  Lord 
Rayleigh  and  Professor  Abbe,  make  a  long  list  of  notable 
achievements. 

To  these  may  be  added  his  perfecting  of  the  achromatic  tele- 
scope, the  equatorial  mounting  and  its  clockwork  drive,  the 
improvement  of  the  heliometer,  the  invention  of  the  stage  mi- 


THE  MODERN  TELESCOPE  39 

crometer,  several  types  of  ocular  micrometers,   and  the  auto- 
matic ruling  engine. 

He  died  at  the  height  of  his  creative  powers  June  7,  1826,  and 
lies  buried  at  Munich  under  the  sublime  ascription,  by  none 
better  earned,  Approximavit  Sidera. 

From  Fraunhofer's  time,  at  the  hands  of  Merz  his  immediate 
successor,  Cauchoix  in  France,  and  Tully  in  England,  the  achro- 
matic refractor  steadily  won  its  way.  Reflecting  telescopes, 
despite  the  sensational  work  of  Lord  Rosse  on  his  6-foot  mirror  of 
53  feet  focus  (unequalled  in  aperture  until  the  6-foot  of  the 
Dominion  Observatory  seventy  years  later),  and  the  even  more 
successful  instrument  of  Mr.  Lassell  (4  feet  aperture,  39  feet 
focus),  were  passing  out  of  use,  for  the  reason  already  noted, 
that  repolishing  meant  refiguring  and  the  user  had  to  be  at  once 
astronomer  and  superlatively  skilled  optician. 

These  large  specula,  too,  were  extremely  prone  to  serious  flexure 
and  could  hardly  have  been  used  at  all  except  for  the  equili- 
brating levers  devised  by  Thomas  Grubb  about  1834,  and  used 
effectively  on  the  Rosse  instrument.  These  are  in  effect  a  group 
of  upwardly  pressing  counterbalanced  planes  distributing  among 
them  the  downward  component  of  the  mirror's  weight  so  as  to 
keep  the  figure  true  in  any  position  of  the  tube. 

Such  was  the  situation  in  the  50's  of  the  last  century,  when  the 
reflector  was  quite  unexpectedly  pushed  to  the  front  as  a  practical 
instrument  by  almost  simultaneous  activity  in  Germany  and 
France.  The  starting  point  in  each  was  Liebig's  simple  chemical 
method  of  silvering  glass,  which  quickly  and  easily  lays  on  a  thin 
reflecting  film  capable  of  a  beautiful  polish. 

The  honor  of  technical  priority  in  its  application  to  silvering 
telescope  specula  worked  in  glass  belongs  to  Dr.  Karl  August 
Steinheil  (1801-1870)  who  produced  about  the  beginning  of 
1856  an  instrument  of  4-inch  aperture  reported  to  have  given 
with  a  power  of  100  a  wonderfully  good  image.  The  publication 
was  merely  from  a  news  item  in  the  " Allgemeine  Zeitung" 
of  Augsburg,  March  24,  1856,  so  it  is  little  wonder  that  the 
invention  passed  for  a  time  unnoticed. 

Early  the  next  year,  Feb.  16, 1857,  working  quite  independently, 
exactly  the  same  thing  was  brought  before  the  French  Academy 
of  Sciences  by  another  distinguished  physicist,  Jean  Bernard 
Leon  Foucault,  immortal  for  his  proof  of  the  earth's  rotation  by 


40 


THE  TELESCOPE 


Fig.   24. — Dr.  Karl  August  Steinheil. 


Fig.  25. — Jean  Bernard  Leon  Foueault. 
The  Inventors  of  the  Silver-on-Glass  Reflector, 


THE  MODERN  TELESCOPE 


41 


the  pendulum  experiment,  his  measurement  of  the  velocity  of 
light,  and  the  discovery  of  the  electrical  eddy  currents  that  bear 
his  name. 

To  Foucault,  chiefly,  the  world  owes  the  development  of  the 
modern  silver-on-glass  reflector,  for  not  being  a  professional 
optician  he  had  no  hesitation  in  making  public  his  admirable 
methods  of  working  and  testing,  the  latter  now  universally 
employed.     It  is  worth  noting  that  his  method  of  figuring  was, 


Fig.  26. — Early  Foucault  Reflector. 

physically,  exactly  what  Jesse  Ramsden  (1735-1800)  had  pointed 
out  in  1779,  (Phil.  Tr.  1779,  427)  geometrically.  One  of 
Foucault's  very  early  instruments  mounted  equatorially  by 
Secretan  is  shown  in  Fig.  26. 

The  immediate  result  of  the  admirable  work  of  Steinheil  and 
Foucault  was  the  extensive  use  of  the  new  reflector,  and  its  rapid 
development  as  a  convenient  and  practical  instrument,  especially 
in  England  in  the  skillful  hands  of  With,  Browning,  and  Calver. 
Not  the  least  of  its  advantages  was  its  great  superiority  over  the 
older  type  in  light-grasp,  silver  being  a  better  reflector  than  specu- 


42 


THE  TELESCOPE 


lum  metal  in  the  ratio  of  very  nearly  7  to  5.  From  this  time  on 
both  refractors  and  reflectors  have  been 
fully  available  to  the  user  of  telescopes. 

In  details  of  construction  both  have 
gained  somewhat  mechanically.  As  we 
have  seen,  tubes  were  often  of  wood,  and 
not  uncommonly  the  mountings  also.  At 
the  present  time  metal  work  of  every  kind 
being  more  readily  available,  tubes  and 
mountings  of  telescopes  of  every  size  are 
quite  universally  of  metal,  save  for  the 
tripod-legs  of  the  portable  instruments. 
The  tubes  of  the  smaller  refractors,  say  3  to 
5  inches  in  aperture,  are  generally  of  brass, 
though  in  high  grade  instruments  this  is 
rapidly  being  replaced  by  aluminum,  which 
saves  considerable  weight.  Tubes  above  5 
or  6  inches  are  commonly  of  steel,  painted 
or  lacquered.  The  beautifully  polished 
brass  of  the  smaller  tubes,  easily  damaged 
and  objectionably  shiny,  is  giving  way  to 
a  serviceable  matt  finish  in  hard  lacquer. 
Mountings,  too,  are  now  more  often  in 
iron  and  steel  or  aluminum  than  in  brass, 
the  first  named  quite  universally  in  the 
working  parts,  for  which  the  aluminum  is 
rather  soft. 

The  typical  modern  refractor,  even  of 
modest  size,  is  a  good  bit  more  of  a  machine 
than  it  looks  at  first  glance.  In  principle 
it  is  outlined  in  Fig,  5,  in  practice  it  is 
much  more  complex  in  detail  and  requires 
the  nicest  of  workmanship.  In  fact  if  one 
were  to  take  completely  apart  a  well- 
made  small  refractor,  including  its  optical 
and  mechanical  parts  one  would  reckon  up 
some  30  to  40  separate  pieces,  not  counting 
screws,  all  of  which  must  be  accurately 
fitted  and  assembled  if  the  instrument  is 
to  work  properly. 

Fig.  27   shows   such    an   instrument   in 


[ 


Ci,    m 


THE  MODERN  TELESCOPE  43 

section  from  end  to  end,  as  one  would  find  it  could  he  lay  it  open 
longitudinally. 

A  is  the  objective  cap  covering  the  objective  B  in  its  adjustable 
cell  C,  which  is  squared  precisely  to  the  axis  of  the  main  tube  D. 
Looking  along  this  one  finds  the  first  of  the  diaphragms,  E. 

These  are  commonly  3  to  6  in  number  spaced  about  equally 
down  the  tube,  and  are  far  more  important  than  they  look. 
Their  function  is  not  to  narrow  the  beam  of  light  that  reaches 
the  ocular,  but  to  trap  light  which  might  enter  the  tube  obliquely 
and  be  reflected  from  its  sides  into  the  ocular,  filling  it  with  stray 
glare. 

No  amount  of  simple  blackening  will  answer  the  purpose,  for 
even  dead  black  paint  such  as  opticians  use  reflects  at  very  oblique 
incidence  quite  10  to  20  per  cent  of  the  beam.  The  impor- 
tance of  both  diaphragms  and  thorough  blackening  has  been 
realized  for  at  least  a  century  and  a  half,  and  one  can  hardly 
lay  too  much  stress  upon  the  matter 

The  diaphragms  should  be  so  proportioned  that,  when  looking 
up  the  tube  from  the  edge  of  an  aperture  of  just  the  size  and  posi- 
tion of  the  biggest  lens  in  the  largest  eye-piece,  no  part  of  the 
edge  of  the  objective  is  cut  off,  and  no  part  of  the  side  of  the 
tube  is  visible  beyond  the  nearest  diaphragm. 

Going  further  down  the  tube  past  a  diaphragm  or  two  one 
comes  to  the  clamping  screws  F.  These  serve  to  hold  the  instru- 
ment to  its  mounting.  They  may  be  set  in  separate  bases 
screwed  in  place  on  the  inside  of  the  tube,  or  may  be  set  in  the 
two  ends  of  a  lengthwise  strap  thus  secured.  They  are  placed  at 
the  balance  point  as  nearly  as  may  be,  generally  nearer  the  eye 
end  than  the  objective. 

Then,  after  one  or  more  diaphragms,  comes  the  guide  ring  G, 
which  steadies  the  main  draw  tube  H,  and  the  rack  /  by  which  it 
is  moved  for  the  focussing  in  turning  the  milled  head  of  the  pinion 
J.  The  end  ring  K  of  the  main  tube  furnishes  the  other  bearing 
of  H,  and  both  G  and  K  are  commonly  recessed  for  accurately 
fitted  cloth  lining  rings  L,  L,  to  give  the  draw  tube  the  necessary 
smoothness  of  motion. 

For  the  same  reason  I  and  J  have  to  be  cut  and  fitted  with  the 
utmost  exactness  so  as  to  work  evenly  and  without  backlash. 
H  is  fitted  at  its  outer  end  with  a  slide  ring  and  tube  M,  generally 
again  cloth  lined  to  steady  the  sliding  eye-piece  tube  N.  This  is 
terminated  by  the  spring  collar  0,  in  which  fits  the  eye-piece  P, 


44 


THE  TELESCOPE 


generally  of  the  two  lens  form;  and  finally  comes  the  eye-piece  cap 
Q  set  at  the  proper  distance  from  the  eye  lens  and  with  an  aper- 
ture of  carefully  determined  size. 

One  thus  gets  pretty  well  down  in  the  alphabet  without  going 
much  into  the  smaller  details  of  construction.  Both  objective 
mount  and  ocular  are  somewhat  complex  in  fact,  and  the  former 
is  almost  always  made  adjustable  in  instruments  of  above  3  or 
4  inches  aperture,  as  shown  in  Fig.  28,  the  form  used  by  Cooke,  the 
famous  maker  of  York,  England.  Unless  the  optical  axis  of  the 
objective  is  true  with  the  tube  bad  images  result. 


Fig.  28. — Adjustable  Cell  for  Objective. 

To  the  upper  end  of  the  tube  is  fitted  a  flanged  counter-cell 
c,  to  an  outward  flange  /,  tapped  for  3  close  pairs  of  adjustng 
screws  as  Si,  Sn  spaced  at  120°  apart.  The  objective  cell  itsielf, 
h,  is  recessed  for  the  objective  which  is  held  in  place  by  an 
interior  or  exterior  ring  d.  The  two  lenses  of  the  achromatic 
objective  are  usually  very  shghtly  separated  by  spacers,  either 
tiny  bits  of  tinfoil  120°  apart,  or  a  very  thin  ring  with  its  upper 
edge  cut  down  save  at  3  points. 

This  precaution  is  to  insure  that  the  lenses  are  quite  uniformly 
supported  instead  of  touching  at  uncertain  points,  and  quite 
usually  the  pair  as  a  whole  rests  below  on  three  corresponding 
spacers.  Of  each  pair  of  adjusting  screws  one  as  1  in  the  pair  Su 
is  threaded  to  push  the  counter  cell  out,  the  adjacent  one,  2,  to  pull 
it  in,  so  that  when  adjustment  is  made  the  objective  is  firmly  held. 
Of  the  lenses  that  form  the  objective,  the  concave  flint  is  com- 
monly at  the  rear  and  the  convex  crown  in  front. 


THE  MODERN  TELESCOPE 


45 


At  the  eye  end  the  ocular  ordinarily  consists  of  two  lenses 
each  burnished  into  a  brass  screw  ring,  a  tube,  flange,  cap,  and 
diaphragm  arranged  as  shown  in  Fig.  29.  There  are  many 
varieties  of  ocular  as  will  presently  be  shown,  but  this  is  a  typical 
form.  Figure  30  shows  a  complete  modern  refractor  of  four 
inches  aperture  on  a  portable  equatorial  stand  with  slow  motion 
in  right  ascension  and  diagonal  eye  piece. 

Reflectors,  used  in  this  country  less  than  they  deserve,  are, 
when  properly  mounted,  likewise  possessed  of  many  parts.  The 
smaller  ones,  such  as  are  likely  to  come  into  the  reader's  hands, 
are  almost  always  in  the  Newtonian  form,  with  a  small  oblique 
mirror  to  bring  the  image  outside  the  tube. 


Fig.  29. — The  Eye-Piece  and  its  Fittings. 


The  Gregorian  form  has  entirely  vanished.  Its  only  special 
merit  was  its  erect  image,  which  gave  it  high  value  as  a  terres- 
trial telescope  before  the  days  of  achromatics,  but  from  its 
construction  it  was  almost  impossible  to  keep  the  field  from  being 
jflooded  with  stray  light,  and  the  achromatic  soon  displaced  it. 
The  Cassegranian  construction  on  the  other  hand,  shorter  and 
with  aberrations  much  reduced,  has  proved  important  for  obtain- 
ing long  equivalent  focus  in  a  short  mount,  and  is  almost  uni- 
versally applied  to  large  reflectors,  for  which  a  Newtonian  mirror 
is  also  generally  provided. 

Figure  31  shows  in  section  a  typical  reflector  of  the  Newtonian 
form.  Here  A  is  the  main  tube,  fitted  near  its  outer  end  with  a 
ring  B  carrying  the  small  elliptical  mirror  C,  which  is  set  at  45° 
to  the  axis  of  the  tube.  At  the  bottom  of  the  tube  is  the  para- 
bolic main  mirror  Z),  mounted  in  its  cell  E.  Just  opposite  the 
45°  small  mirror  is  a  hole  in  the  tube  to  which  is  fitted  the  eye 


46 


THE  TELESCOPE 


piece  mounting  F,  carrying  the  eye-piece  G,  fitted  to  a  spring 
collar  H^  screwed  into  a  draw  tube  I,  sliding  in  its  mounting  and 
brought  to  focus  by  the  rack-and-pinion  J. 


Fig.  30. — Portable  Equatorial  Refractor  (Brashear). 


At  K,  K,  are  two  rings  fixed  to  the  tube  and  bearing  smoothly 
against  the  rings  L  L  rigidly  fixed  to  the  bar  M  carried  by  the 
polar  axis  of  the  mount.  The  whole  tube  can  therefore  be  rotated 
about  its  axis  so  as  to  bring  the  eye  piece  into  a  convenient  posi- 


THE  MODERN  TELESCOPE 


47 


48  THE  TELESCOPE 

tion  for  observation.     One  or  more  handles,  N,  are  provided  for 
this  purpose. 

Brackets  shown  in  dotted  lines  at  0,  0,  carry  the  usual  finder, 
and  a  hinged  door  P  near  the  lower  end  of  the  tube  enables  one  to 
remove  or  replace  the  close  fitting  metal  cover  that  protects  the 
main  mirror  when  not  in  use.  Similarly  a  cover  is  fitted  to  the 
small  mirror,  easily  reached  from  the  upper  end  of  the  tube.     The 


Fig.  32. — Reflector  with  Skeleton  Tube  (Brashear). 

proportions  here  shown  are  approximately  those  commonly 
found  in  medium  sized  instruments,  say  7  to  10  inches  aperture. 
The  focal  ratio  is  somewhere  about  F/Q,  the  diagonal  mirror  is 
inside  of  focus  by  about  the  diameter  of  the  main  mirror,  and 
its  minor  axis  is  from  3^^  to  3-^  that  diameter. 

Note  that  the  tube  is  not  provided  with  diaphragms.  It 
is  merely  blackened  as  thoroughly  as  possible,  although  stray 
light  is  quite  as  serious  here  as  in  a  refractor.     One  could  fit 


i:ms^^Ns;^?;^^^^y:^^^;^^m^ 


THE  MODERN  TELESCOPE  49 

diaphragms  effectively  only  in  a  tube  of  much  larger  diam- 
eter than  the  mirror,  which  would  be  inconvenient  in  many 
ways. 

A  much  better  way  of  dealing  with  the  difficulty  is  shown  in  Fig. 
32  in  which  the  tube  is  reduced  to  a  skeleton,  a  construction  com- 
mon in  large  instruments.  Nothing  is  blacker  than  a  clear 
opening  into  the  darkness  of  night,  and  in  addition  there  can  be 
no  localized  air  currents,  which  often  injure  definition  in  an  ordi- 
nary tube. 

Instruments  by  different  makers  vary  somewhat  in  detail.     A 
good  type  of  mirror  mounting  is 
that  shown  in  Fig.  33,  and   used 
for  man  yyears  past  by  Browning, 
one  of  the  famous  English  makers. 

Here  the  mirror  A,  the  back  of  '^d  ^^d 

which  is  made  accurately  plane,  ^  ^ 

is  seated  in  its  counter-cell  B,  of  ^^°-  ^^• 

which  a  wide  annulus  F,  F,  is  also  a  good  plane,  and  is  lightly  held 
in  place  by  a  retaining  ring.  This  counter  cell  rests  in  the  outer 
cell  C  on  three  equidistant  studs  regulate  d  by  the  concentric 
push-and-puU  adjusting  screws  D,  D,  E,  E.  The  outer  cell 
may  be  solid,  or  a  skeleton  for  lightness  and  better  equalization 
of  temperature. 

Small  specula  may  be  well  supported  on  any  flat  surface  sub- 
stantial enough  to  be  thoroughly  rigid,  with  one  or  more  thick- 
nesses of  soft,  thick,  smooth  cloth  between,  best  of  all  Brussels 
carpet.  Such  was  the  common  method  of  support  in  instruments 
of  moderate  dimensions  prior  to  the  day  of  glass  specula.  Sir 
John  Herschel  speaks  of  thus  carrying  specula  of  more  than  a 
hundred-weight,  but  something  akin  to  Browning's  plan  is 
generally  preferable. 

There  is  also  considerable  variety  in  the  means  used  for  sup- 
porting the  small  mirror  centrally  in  the  tube.  In  the  early 
telescopes  it  was  borne  by  a  single  stiff  arm  which  was  none  too 
stiff  and  produced  by  diffraction  a  long  diametral  flaring  ray  in 
the  images  of  bright  stars. 

A  great  improvement  was  introduced  by  Browning  more  than 
a  half  century  ago,  in  the  support  shown  in  Fig.  34.  Here  the 
ring  A,  {B,  Fig.  31)  carries  three  narrow  strips  of  thin  spring 
steel,  B,  extending  radially  inward  to  a  central  hub  which  carries 
the  mirror  D,  on  adjusting  screws  E.     Outside  the  ring  the  ten- 

4 


50 


THE  TELESCOPE 


sion  screws  C  enable  the  mirror  to  be  accurately  centered  and 
held  in  place.  Rarely,  the  mirror  is  replaced  by  a  totally  reflect- 
ing right  angled  prism  which  saves  some  light,  but  unless  for 
small  instruments  is  rather  heavy  and  hard  to  obtain  of  the  req- 
uisite quality  and  precision  of  figure.  A  typical  modern  reflector 
by  Brashear,  of  6  inches  aperture,  is  shown  in  Fig.  35,  complete 
with  circles  and  driving  clock,  the  latter  contained  in  the  hollow 
iron  pier,  an  arrangement  usual  in  American-made  instruments. 


c  c 

Fig.  34. — Support  of  Diagonal  Mirror  (Browning.) 


Recent  reflectors,  particularly  in  this  country,  have  four  sup- 
porting strips  instead  of  three,  which  gives  a  little  added  stiffness, 
and  produces  in  star  images  but  four  diffraction  rays  instead  of 
the  six  produced  by  the  three  strip  arrangement,  each  strip 
giving  a  diametral  ray. 

In  some  constructions  the  ring  A  is  arranged  to  carry  the  eye- 
piece fittings,  placed  at  the  very  end  of  the  tube  and  arranged  for 
rotating  about  the  optical  axis  of  the  telescope.  This  allows  the 
ocular  to  be  brought  to  any  position  without  turning  the  whole 
tube.  In  small  instruments  a  fixed  eye-piece  can  be  used  without 
much  inconvenience  if  located  on  the  north  side  of  the  tube  (in 
moderate  north  latitudes). 

Reflectors  are  easily  given  a  much  greater  relative  aperture 
than  is  practicable  in  a  single  achromatic  objective.  In  fact 
they  are  usually  given  apertures  of  F/5  to  F/8  and  now  and  then 
are  pushed  to  or  even  below  F/S.  Such  mirrors  have  been 
successfully  used  for  photography;^  and  less  frequently  for  visual 
observation,  mounted  in  the  Cassegranian  form,  which  commonly 
increases  the  virtual  focal  length  at  least  three  or  four  times.  A 
telescope  so  arranged,  with  an  aperture  of  a  foot  or  more  as  in 

1  An  F/3  mirror  of  Im  aperture  by  Zeiss  was  installed  in  the  observatory 
at  Bergedorf  in  1911,  and  a  similar  one  by  Schaer  is  mounted  at  Carre,  near 
Geneva. 


THE  MODERN  TELESCOPE 


51 


some  recent  examples,   makes   a   very   powerful   and   compact 
instrument. 

This  is  the  form  commonly  adopted  for  the  large  reflectors  of 


Fig.  35. — Small  Equatorially  Mounted  Reflector. 


recent  construction,  a  type  being  the  60-inch  telescope  of  the 
Mount  Wilson  Observatory  of  which  the  primary  focus  is  253^ 
feet  and  the  ordinary  equivalent  focus  as  a  Cassegranian  80 
feet. 


52  THE  TELESCOPE 

Comparatively  few  small  reflectors  have  been  made  or  used 
in  the  United  States,  although  the  climatic  conditions  here  arc 
more  favorable  than  in  England,  where  the  reflector  originated 
and  has  been  very  fully  developed.  The  explanation  may  he 
in  our  smaller  number  of  non-professional  active  astronomers 
who  are  steadily  at  observational  work,  and  can  therefore  use 
reflectors  to  the  best  advantage. 

The  relative  advantages  of  refractors  and  reflectors  have 
long  been  a  matter  of  acrimonious  dispute.  In  fact,  more  of  the 
genuine  odium  theologicwm  has  gone  into  the  consideration  of  this 
matter  than  usually  attaches  to  differences  in  scientific  opinion. 
A  good  many  misunderstandings  have  been  due  to  the  fact  that 
until  recently  few  observers  were  practically  familiar  with  both 
instruments,  and  the  professional  astronomer  was  a  little  inclined 
to  look  on  the  reflector  as  fit  only  for  amateurs.  The  comparison 
is  somewhat  clarified  at  present  by  the  fact  that  the  old  speculum 
metal  reflector  has  passed  out  of  use,  and  the  case  now  stands  as 
between  the  ordinary  refracting  telescope  such  as  has  just  been 
described,  and  the  silver-on-glass  reflector  discussed  immediately 
thereafter. 

The  facts  in  the  case  are  comparatively  simple.  Of  two 
telescopes  having  the  same  clear  aperture,  one  a  reflector  and  the 
other  a  refractor,  each  assumed  to  be  thoroughly  well  figured,  as  it 
can  be  in  fact  today,  the  theoretical  resolving  power  is  the  same, 
for  this  is  determined  merely  by  the  aperture,  so  that  the  only 
possible  difference  between  the  two  would  be  in  the  residual  im- 
perfection in  the  performance  of  the  refractor  due  to  its  not  being 
perfectly  achromatic.  This  difference  is  substantially  a  negligi- 
ble one  for  many,  but  not  all,  purposes. 

Likewise,  the  general  definition  of  the  pair,  assuming  first- 
class  workmanship,  would  be  equal.  Of  the  two,  the  single 
surface  of  the  mirror  is  somewhat  more  difficult  to  figure  with  the 
necessary  precision  than  is  any  single  surface  of  the  refractor,  but 
reflectors  can  be,  and  are,  given  so  perfect  a  parabolic  figure  that 
the  image  is  in  no  wise  inferior  to  that  produced  by  the  best 
refractors,  and  the  two  types  of  telescopes  will  stand  under 
favorable  circumstances  the  same  proportional  magnifying 
powers. 

The  mirror  is  much  more  seriously  affected  by  changes  of 
temperature  and  by  flexure  than  is  the  objective,  since  in  the 
former  case  the  successive  surfaces  of  the  two  lenses  in  the  achro- 


THE  MODERN  TELESCOPE  53 

matic  combination  to  a  considerable  extent  compensate  each 
other's  shght  changes  of  curvature,  which  act  only  by  still  slighter 
changes  of  refraction,  while  the  mirror  surface  stands  alone  and 
any  change  in  curvature  produces  double  the  defect  on  the  re- 
flected ray. 

It  is  therefore  necessary,  as  we  shall  see  presently,  to  take 
particular  precautions  in  working  with  a  reflecting  telescope, 
which  is,  so  to  speak,  materially  more  tender  as  regards  external 
conditions  than  the  refractor.  As  regards  light-grasp,  the  power 
of  rendering  faint  objects  visible,  there  is  more  room  for  honest 
variety  of  opinion.  It  was  often  assumed  in  earlier  days  that  a 
reflector  was  not  much  brighter  than  a  refractor  of  half  the 
aperture,  i.e.,  of  one  quarter  the  working  area. 

This  might  have  been  true  in  the  case  of  an  old  speculum  metal 
reflector  in  bad  condition,  but  is  certainly  a  libel  on  the  silver-on- 
glass  instrument,  which  Foucault  on  the  other  hand  claimed  to  be, 
aperture  for  aperture,  brighter  than  the  refractor.  Such  a 
relation  might  in  fact  temporarily  exist,  but  it  is  far  from 
typical. 

The  real  relation  depends  merely  on  the  light  losses  demon- 
strably occurring  in  the  two  types  of  telescopes.  These  are  now 
quite  well  known.  The  losses  in  a  refractor  are  those  due  to 
absorption  of  hght  in  the  two  lenses,  plus  those  due  to  the  four 
free  surfaces  of  these  lenses.  The  former  item  in  objectives  of 
moderate  size  aggregates  hardly  more  than  2  to  3  per  cent. 
The  latter,  assuming  the  polish  to  be  quite  perfect,  amount  to 
18  to  20  per  cent  of  the  incident  light,  for  the  glasses  com- 
monly used. 

The  total  light  transmitted  is  therefore  not  over  80  per  cent 
of  the  whole,  more  often  somewhat  under  this  figure.  For 
example,  a  test  by  Steinheil  of  one  of  Fraunhofer's  refractors 
gave  a  transmission  of  78  per  cent,  and  other  tests  show  similar 
results. 

The  relation  between  the  light  transmitted  by  glass  of  various 
thickness  is  very  simple.  If  unit  thickness  transmits  m  per  cent 
of  the  incident  light  then  n  units  in  thickness  will  pass  m"  per 
cent.  Thus  if  one  half  inch  passes  .98,  two  inches  will  transmit 
.98^,  or  .922.  Evidently  the  bigger  the  objective  the  greater  the 
absorptive  loss.  If  the  loss  by  reflection  at  a  single  surface 
leaves  m  per  cent  to  be  transmitted  then  n  surfaces  will  transmit 
m".     And  m  being  usually  about  .95,  the  four  surfaces  of  an  objec- 


54  THE  TELESCOPE 

tive  let  pass  nearly  .815,  and  the  thicker  objective  as  a  whole 
transmits  approximately  75  per  cent. 

As  to  the  reflector  the  whole  relation  hinges  on  the  coefficient 
of  reflection  from  a  silvered  surface,  under  the  circumstances  of 
the  comparison. 

In  the  case  of  a  reflecting  telescope  as  a  whole,  there  are 
commonly  two  reflections  from  silver  and  if  the  coefficient  of 
reflection  is  m  then  the  total  light  reflected  is  Is^.  Now  the 
reflectivity  of  a  silver-on-glass  film  has  been  repeatedly  measured. 
Chant  Ap.  J.  21,  211)  found  values  slightly  in  excess  of  95 
per  cent,  Rayleigh  (Sci.  Papers  2,  4)  got  93.9,  Zeiss  (Landolt  u. 
Bornstein,  Tabellen)  about  93.0  for  light  of  average  wave 
length. 

Taking  the  last  named  value,  a  double  reflection  would  return 
substantially  86.5  per  cent  of  the  incident  light.  No  allowance 
is  here  made  for  any  effect  of  selective  reflection,  since  for  the 
bright  visual  rays,  which  alone  we  are  considering,  there  is  very 
slight  selective  effect.  In  the  photographic  case  it  must  be 
taken  into  account,  and  the  absorption  in  glass  becomes  a 
serious  factor  in  the  comparison,  amounting  for  the  photographic 
rays  to  as  much  as  30  to  40  per  cent  in  large  instruments.  Now 
in  comparing  reflector  and  refractor  one  must  subtract  the 
light  stopped  by  the  small  mirror  and  its  supports,  commonly 
from  5  to  7  per  cent.  One  is  therefore  forced  to  the  conclusion 
that  with  silver  coatings  fresh  and  very  carefully  polished 
reflector  and  refractor  will  show  for  equal  aperture  equal  light 
grasp. 

But  as  things  actually  go  even  fresh  silver  films  are  quite  often 
below  .90  in  reflectivity  and  in  general  tarnish  rather  rapidly, 
so  that  in  fact  the  reflector  falls  below  the  refractor  by  just  about 
the  amount  by  which  the  silver  films  are  out  of  condition.  For 
example  Chant  (loc.  cit.)  found  after  three  months  his  reflectivity 
had  fallen  to  .69.  A  mirror  very  badly  tarnished  by  fifteen  weeks 
of  exposure  to  dampness  and  dust,  uncovered,  was  found  by  the 
writer  down  to  a  scant  .40. 

The  line  of  Fig.  36  shows  the  relative  equivalent  apertures  of 
refractors  corresponding  to  a  10  inch  reflector  at  coefficients  of 
reflection  for  a  single  silvered  surface  varying  from  .95  to  .50  at 
which  point  the  film  would  be  so  evidently  bad  as  to  require  im- 
mediate renewal.  The  relation  is  obviously  linear  when  the 
transmission  of  the  objective  is,  as  here,  assumed  constant.     The 


THE  MODERN  TELESCOPE 


55 


estimates  of  skilled  observers  from  actual  comparisons  fall  in  well 
with  the  line,  showing  reflectivities  generally  around  .80  to  .85 
for  well  polished  films  in  good  condition. 

The  long  and  short  of  the  situation  is  that  a  silvered  reflector 
deteriorates  and  at  intervals  varying  from  a  few  months  to  a  year 
or  two  depending  on  situation,  climate,  and  usage,  requires 
repolishing  or  replacement  of  the  film.  This  is  a  fussy  job,  but 
quickly  done  if  everything  goes  well. 

As  to  working  field  the  reflector  as  ordinarily  proportioned  is  at 
a  disadvantage  chiefly  because  it  works  at  i^/5  or  F/6  instead  of 


\ 

\ 

\ 

8 

7 

\ 

\ 

^ 

N, 

\ 

\ 

0 
5 

\ 

\ 

100  .80  .60  .40 

Reflectivity 

Fig.  36. — Relative  Light-grasp  of  Ref.ector  and  Refractor. 


at  ii^/15.  At  equal  focal  ratios  there  is  no  substantial  difference 
between  reflector  and  refractor  in  this  respect,  unless  one  goes 
into  special  constructions,  as  in  photographic  telescopes. 

In  two  items,  first  cost  and  convenience  in  observing,  the 
reflector  has  the  advantage  in  the  moderate  sizes.  Roughly, 
the  reflector  simply  mounted  costs  about  one  half  to  a  quarter  the 
refractor  of  equal  light  grasp  and  somewhat  less  resolving  power, 
the  discrepancy  getting  bigger  in  large  instruments  (2  feet  aperture 
and  upwards). 


56  THE  TELESCOPE 

As  to  ease  of  observing,  the  small  refractor  is  a  truly  neck- 
wringing  instrument  for  altitudes  above  45°  or  thereabouts,  just 
the  situatio^i  in  which  the  equivalent  reflector  is  most  convenient. 
In  considering  the  subject  of  mounts  these  relations  will  appear 
more  clearly. 

Practically  the  man  who  is  observing  rather  steadily  and  can 
give  his  telescope  a  fixed  mount  can  make  admirable  use  of  a 
reflector  and  will  not  find  the  perhaps  yearly  or  even  half  yearly 
re-silvering  at  all  burdensome  after  he  has  acquired  the  knack — 
chiefly  cleanliness  and  attention  to  detail. 

If,  like  many  really  enthusiastic  amateurs,  he  can  get  only  an 
occasional  evening  for  observing,  and  from  circumstances  has  to 
use  a  portable  mount  set  up  on  his  lawn,  or  even  roof,  when 
fortune  favors  an  evening's  work,  he  will  find  a  refractor  always 
in  condition,  easy  to  set  up,  and  requiring  a  minimum  of  time  to 
get  into  action.  The  reflector  is  much  the  more  tender  instru- 
ment, with,  however,  the  invaluable  quality  of  precise  achroma- 
tism, to  compensate  for  the  extra  care  it  requires  for  its  best 
performance.  It  suffers  more  than  the  refractor,  as  a  rule,  from 
scattered  light,  for  imperfect  polish  of  the  film  gives  a  field 
generally  presenting  a  brighter  background  than  the  field  of  a 
good  objective.  After  all  the  preference  depends  greatly  on  the 
use  to  which  the  telescope  is  to  be  put.  For  astrophysical  work 
in  general.  Professor  George  E.  Hale,  than  whom  certainly  no  one 
is  better  qualified  to  judge,  emphatically  endorses  the  refiector. 
Most  large  observatories  are  now-a-days  equipped  with  both 
refractors  and  reflectors. 


CHAPTER  III 
OPTICAL  GLASS  AND  ITS  WORKING 

Glass,  one  of  the  most  remarkable  and  useful  products  of 
man's  devising,  had  an  origin  now  quite  lost  in  the  mists  of 
antiquity.  It  dates  back  certainly  near  a  thousand  years  before 
the  Christian  era,  perhaps  many  centuries  more.  Respecting  its 
origin  there  are  only  traditions  of  the  place,  quite  probably  Syria, 
and  of  the  accidental  melting  together  of  sand  and  soda.  The 
product,  sodium  silicate,  readily  becomes  a  liquid,  i.e.,  "water- 
glass,"  but  the  elder  Pliny,  who  tells  the  story,  recounts  the  later 
production  of  a  stable  vitreous  body  by  the  addition  of  a  mineral 
which  was  probably  a  magnesia  limestone. 

This  combination  would  give  a  good  permanent  glass,  whether 
the  story  is  true  or  not,  and  very  long  before  Pliny's  time  glass 
was  made  in  great  variety  of  composition  and  color.  In  fact 
in  default  of  porcelain  glass  was  used  in  Roman  times  relatively 
more  than  now.  But  without  knowledge  of  optics  there  was  no 
need  for  glass  of  optical  quality,  it  was  well  into  the  Renaissance 
before  its  manufacture  had  reached  a  point  where  anything  of  the 
sort  could  be  made  available  even  in  small  pieces,  and  it  ig  barely 
over  a  century  since  glass-making  passed  beyond  the  crudest 
empiricism. 

Glass  is  substantially  a  solid  solution  of  silica  with  a  variety  of 
metallic  oxides,  chiefly  those  of  sodium,  potassium,  calcium  and 
lead,  sometimes  magnesium,  boron,  zinc,  barium  and  others. 

By  itself  silica  is  too  refractory  to  work  easily,  though 
silica  glass  has  some  very  valuable  properties,  and  the  alkaline 
oxides  in  particular  serve  as  the  fluxes  in  common  use.  Other 
oxides  are  added  to  obtain  various  desired  properties,  and  some 
impurities  may  go  with  them. 

The  melted  mixture  is  thus  a  somewhat  complex  solution  con- 
taining frequently  half  a  dozen  ingredients.  Each  has  its  own 
natural  melting  and  vaporizing  point,  so  that  while  the  blend 
remains  fairly  uniform  it  may  tend  to  lose  some  constituent  while 
molten,  or  in  cooling  to  promote  the  crystaUization  of  another,  if 
held  too  near  its  particular  freezing  point.  Some  combinations  are 
more  likely  to  give  trouble  from  this  cause  than  others,  and  while 

57 


58  THE  TELESCOPE 

a  very  wide  variety  of  oxides  can  be  coerced  into  solution  with 
silica,  a  comparatively  limited  number  produce  a  homogeneous 
and  colorless  glass  useful  for  optical  purposes. 

Many  mixtures  entirely  suitable  for  common  commercial  pur- 
poses are  out  of  the  question  for  lens  making,  through  tendency 
to  surface  deterioration  by  weathering,  lack  of  homogeneous 
quality,  or  objectionable  coloration.  A  very  small  amount  of 
iron  in  the  sand  used  at  the  start  gives  the  green  tinge  familiar 
in  cheap  bottles,  which  materially  decreases  the  transparency. 
The  bottle  maker  often  adds  oxide  of  manganese  to  the  mixture, 
which  naturally  of  itself  gives  the  glass  a  pinkish  tinge,  and  so 
apparently  whitens  it  by  compensating  the  one  absorption  by 
another.  The  resulting  glass  looks  all  right  on  a  casual  glance, 
but  really  cuts  off  a  very  considerable  amount  of  light. 

A  further  difficulty  is  that  glass  differs  very  much  in  its  degree 
of  fluidity,  and  its  components  sometimes  seem  to  undergo  mutual 
reactions  that  evolve  persistent  fine  bubbles,  besides  reacting 
with  the  fireclay  of  the  melting  pot  and  absorbing  impurities 
from  it. 

The  molten  glass  is  somewhat  viscous  and  far  from  homogene- 
ous. Its  character  suggests  thick  syrup  poured  into  water,  and 
producing  streaks  and  eddies  of  varying  density.  Imagine  such  a 
mixture  suddenly  frozen,  and  you  have  a  good  idea  of  a  common 
condition  in  glass,  transparent,  but  full  of  striae.  These  are 
frequent  enough  in  poor  window  glass,  and  are  almost  impossible 
completely  to  get  rid  of,  especially  in  optical  glass  of  some  of  the 
most  valuable  varieties. 

The  great  improvement  introduced  by  Guinand  was  constant 
stirring  of  the  molten  mass  with  a  cylinder  of  fire  clay,  bringing 
bubbles  to  the  surface  and  keeping  the  mass  throughly  mixed 
from  its  complete  fusion  until,  very  slowly  cooling,  it  became  too 
viscous  to  stir  longer. 

The  fine  art  of  the  process  seems  to  be  the  exact  combination  of 
temperature,  time,  and  stirring,  suitable  for  each  composition 
of  the  glass.  There  are,  too,  losses  by  volatilization  during 
melting,  and  even  afterwards,  that  must  be  reckoned  with  in  the 
proportions  of  the  various  materials  put  into  the  melting,  and  in 
the  temperatures  reached  and  maintained. 


OPTICAL  GLASS  AND  ITS  WORKING  59 

One  cannot  deduce  accurately  the  percentage  mixture  of  the 
raw  materials  from  an  analysis  of  the  glass,  and  it  is  notorious 
that  the  product  even  of  the  best  manufacturers  not  infrequently 
fails  to  run  quite  true  to  type.  Therefore  the  optical  properties 
of  each  melting  have  carefully  to  be  ascertained,  and  the  product 
listed  either  as  a  very  slight  variant  from  its  standard  type,  or 
as  an  odd  lot,  useful,  but  quite  special  in  properties.  Some  of 
these  odd  meltings  in  fact  have  optical  peculiarities  the  regular 
reproduction  of  which  would  be  very  desirable. 

The_j)urity  of  the  materials  is  of  the  utmost  impoi'tance  in 
producing  high  grade  glass  for  optical  or  other  purposes.  The 
.  silica  is  usually  introduced  in  the  form  of  the  purest  of  white 
sand  carrying  only  a  few  hundredths  of  one  per  cent  of  impurities 
in  the  way  of  iron,  alumina  and  alkali.  The  ordinary  alkalis 
go  in  preferably  as  carbonates,  which  can  be  obtained  of  great 
purity;  arthough  in  most  commercial  glass  the  soda  is  used  in  the 
form  of  ''salt-cake,"  crude  sodium  sulphate. 

Calcium,  magnesium,  and  barium  generally  enter  the  melt  as 
carbonates,  zinc  and  lead  as  oxides.  Alumina,  like  iron,  is  gen- 
erally an  impurity  derived  from  felspar  in  the  sand,  but  occasion- 
ally enters  intentionally  as  pure  natural  felspar,  or  as  chemically 
prepared  hydrate.  A  few  glasses  contain  a  minute  amount  of 
arsenic,  generally  used  in  the  form  of  arsenious  acid,  and  still 
more  rarely  other  elements  enter,  ordinarily  as  oxides. 

Whatever  the  materials,  they  are  commonly  rather  fine  ground 
and  very  thoroughly  mixed,  preferably  by  machinery,  before 
going  into  the  furnaces.  Glass  furnaces  are  in  these  days  com- 
monly gas  fired,  and  fall  into  two  general  classes,  those  in  which 
the  charge  is  melted  in  a  huge  tank  above  which  the  gas  fiames 
play,  and  those  in  which  the  charge  is  placed  in  crucibles  or  pots 
open  or  nearly  closed,  directly  heated  by  the  gas.  In  the  tank 
furnaces  the  production  is  substantially  continuous,  the  active 
melting  taking  place  at  one  end,  where  the  materials  are  intro- 
duced, while  the  clear  molten  glass  flows  to  the  cooler  end  of  the 
tank  or  to  a  cooler  compartment,  whence  it  is  withdrawn  for 
working. 

The  ordinary  method  of  making  optical  glass  is  by  a  modifica- 
tion of  the  pot  process,  each  pot  being  fired  separately  to  permit 
better  regulation  of  the  temperature. 

The  pots  themselves  are  of  the  purest,  of  fire  clay,  of  moderate 
capacity,  half  a  ton  or  so,  and  arched  over  to  protect  the  contents 


60  THE  TELESCOPE 

from  the  direct  play  of  the  gases,  leaving  a  side  opening  sufficient 
for  charging  and  stirring. 

The  fundamental  difference  between  the  making  of  optical 
glass  and  the  ordinary  commercial  varieties  lies  in  the  individual 
treatment  of  each  charge  necessary  to  secure  uniformity  and  regu- 
larity, carried  even  to  the  extent  of  cooling  each  melting  very 
slowly  in  its  own  pot,  which  is  finally  broken  up  to  recover  the 
contents.  The  tank  furnaces  are  under  heat  week  in  and  week 
out,  may  hold  several  hundred  tons,  and  on  this  account  cannot 
so  readily  be  held  to  exactness  of  composition  and  quality. 

The  optical  glass  works,  too,  is  provided  with  a  particularly 
efficient  set  of  preheating  and  annealing  kilns,  for  the  heat  treat- 
ment of  pots  and  glass  must  be  of  the  most  careful  and  thorough 
kind. 

The  production  of  a  melting  of  optical  glass  begins  with  a  very 
gradual  heating  of  the  pot  to  a  bright  red  heat  in  one  of  the  kilns. 
It  is  then  transferred  to  its  furnace  which  has  been  brought  to  a 
similar  temperature,  sealed  in  by  slabs  of  firebrick,  leaving  its 
mouth  easy  of  access,  and  then  the  heat  is  pushed  up  to  near 
the  melting  temperature  of  the  mixture  in  production,  which 
varies  over  a  rather  wide  range,  from  a  moderate  white  heat 
to  the  utmost  that  a  regenerative  gas  furnace  can  conveniently 
produce.  After  the  heating  comes  the  rather  careful  process  of 
charging. 

The  mixture  is  added  a  portion  at  a  time,  since  the  fused 
material  tends  to  foam,  and  the  raw  material  as  a  solid  is  more 
bulky  than  the  fluid.  The  chemical  reactions  as  the  mass  fuses 
are  somewhat  complex.  In  their  simplest  form  they  represent 
the  formation  of  silicates. 

At  high  temperatures  the  silica  acts  as  a  fairly  strong  acid,  and 
decomposes  the  fused  carbonates  of  sodium  and  potassium  with 
evolution  of  gas.  This  is  the  rationale  of  the  fluxing  action  of 
such  alkaline  substances  of  rather  low  melting  point.  Other 
mixtures  act  somewhat  analogously  but  in  a  fashion  commonly 
too  complex  to  follow. 

The  final  result  is  a  thick  solution,  and  the  chief  concern  of 
the  optical  glass  maker  is  to  keep  it  homogeneous,  free  from 
bubbles,  and  as  nearly  colorless  as  practicable.  To  the  first  two 
ends  the  temperature  is  pushed  up  to  gain  fluidity,  and  frequently 
substances  are  added  (e.g.,  arsenic)  which  by  volatility  or  chemical 
effect  tend  to  form  large  bubbles  from  the  entrained  gases,  cap- 


OPTICAL  GLASS  AS D  ITS  WORKIXG 


61 


able  of  clearing  themselves  from  the  fluid  where  fine  bubbles 
would  remain.     For  the  same  purpose  is  the  stirring  process. 

The  stirrer  is  a  hard  baked  cylinder  of  fire  clay  fastened  to  an 
iron  bar.  First  heated  in  the  mouth  of  the  pot,  the  stirrer  is 
plunged  in  the  molten  glass  and  given  a  steady  rotating  motion, 
the  long  bar  being  swivelled  and  furnished  with  a  wooden  handle 
for  the  workman.  This  stirring  is  kept  up  pretty  steadily  while 
the  heat  is  very  slowly  reduced  until  the  mass  is  too  thick  to  man- 
age, the  process  taking,  for  various  mixtures  and  conditions,  from 
three  or  four  hours  to  the  better  part  of  a  day. 

Then  begins  the  careful  and  tedious  process  of  cooling.  Fairty 
rapid  until  the  mass  is  solid  enough  to  prevent  the  formation  of 
fresh  strise,  the  cooling  is  continued  more  slowly,  in  the  furnace 


Fig.  37. — Testing  Optical  Glass  in  the  Rough. 


or  after  removal  to  the  annealing  oven,  until  the  crucible  is  cool 
enough  for  handling,  the  whole  process  generally  taking  a  week  or 
more. 

Then  the  real  trouble  begins.  The  crucible  is  broken  away 
and  there  is  found  a  more  or  less  cracked  mass  of  glass,  sometimes 
badly  broken  up,  again  furnishing  a  clear  lump  weighing  some 
hundreds  of  pounds.  This  glass  is  then  carefully  picked  over  and 
examined  for  flaws,  striae  and  other  imperfections. 

These  can  sometimes  be  chipped  away  mth  more  or  less  break- 
ing up  of  the  mass.  The  inspection  of  the  glass  in  the  raw  is 
facihtated  b}^  the  scheme  shown  in  elevation  Fig.  37.  Here  A  is  a 
tank  with  parallel  sides  of  plate  glass.  In  it  is  placed  B  the 
rough  block  of  glass,  and  the  tank  is  then  filled  with  a  liquid 
which  can  be  brought  to  the  same  refractive  power  as  the  glass, 
as  in  Newton's  disastrous  experiment.  When  equality  is  reached 
for,  say,  yellow  light,  one  can  see  directly  through  the  block,  the 


G2  THE  TELESCOPE 

rays  no  longer  being  refracted  at  its  surface,  and  any  interior 
strise  are  readily  seen  even  in  a  mass  a  foot  or  more  thick.  Before 
adding  the  hquid  a  ray  would  be  skewed,  as  C,  D,  E,  F,  after- 
wards it  would  go  straight  through;  C,  D,  G,  H. 

The  fraction  that  passes  inspection  may  be  found  to  be  from 
much  less  than  a  quarter  to  a  half  of  the  whole.  This  good  glass 
is  then  ready  for  the  next  operation,  forming  and  fine  annealing. 
The  final  form  to  be  reached  is  a  disc  or  block,  and  the  chunks  of 
perfect  glass  are  heated  in  a  kiln  until  plastic,  and  then  moulded 
into  the  required  shapes,  sometimes  concave  or  convex  discs 
suitable  for  small  lenses. 

Then  the  blocks  are  transferred  to  a  kiln  and  allowed  to  cool 
off  very  gradually,  for  several  days  or  weeks  according  to  the  size 
of  the  blocks  and  the  severity  of  the  requirements  they  must 
meet.  In  the  highest  class  of  work  the  annealing  oven  has 
thermostatic  control  and  close  watch  is  kept  by  the  pyrometer. 

It  is  clear  that  the  chance  of  getting  a  large  and  perfect 
chunk  from  the  crucible  is  far  smaller  than  that  of  getting  frag- 
ments of  a  few  pounds,  so  that  the  production  of  a  perfect  disc 
for  a  large  objective  requires  both  skill  and  luck.  Little  wonder 
therefore  that  the  price  of  discs  for  the  manufacture  of  objectives 
increases  substantially  as  the  cube  of  the  diameter. 

The  process  of  optical  glass  making  as  here  described  is  the 
customary  one,  used  little  changed  since  the  days  of  Guinand. 
The  great  advances  of  the  last  quarter  century  have  been  in  the 
production  of  new  varieties  having  certain  desirable  qualities,  and 
in  a  better  understanding  of  the  conditions  that  bring  a  uniform 
product  of  high  quality.  During  the  world  war  the  greatly 
increased  demand  brought  most  extraordinary  activity  in  the 
manufacture,  and  especially  in  the  scientific  study  of  the  problems 
involved,  both  here  and  abroad.  The  result  has  been  a  long 
step  toward  quantity  production,  the  discovery  that  modifica- 
tions of  the  tank  process  could  serve  to  produce  certain  varieties 
of  optical  glass  of  at  least  fair  quality,  and  great  improvements  in 
the  precision  and  rapidity  of  annealing. 

These  last  are  due  to  the  use  of  the  electric  furnace,  the  study 
of  the  strains  during  annealing  under  polarized  light,  and  scien- 
tific pyrometry.  It  is  found  that  cooling  can  be  much  hastened 
over  certain  ranges  of  temperature,  and  the  total  time  required 
very  greatly  shortened.  It  has  also  been  discovered,  thanks  to 
captured  instruments,  that  some  of  the  glasses  commonly  regarded 


OPTICAL  GLASS  AND  ITS  WORKING  63 

as  almost  impossible  to  free  from  bubbles  have  in  fact  yielded  to 
improved  methods  of  treatment. 

Conventionally  optical  glass  is  of  two  classes,  crown  and 
flint.  Originally  the  former  was  a  simple  compound  of  silica 
with  soda  and  potash,  sometimes  also  lime  or  magnesia,  while  the 
latter  was  rich  in  lead  oxide  and  with  less  of  alkali.  The  crown 
had  a  low  index  of  refraction  and  small  dispersion,  the  flint  a 
high  index  and  strong  dispersion.  Crown  glass  was  the  material 
of  general  use,  while  the  flint  glass  was  the  variety  used  in  cut 
glass  manufacture  by  reason  of  its  brilliancy  due  to  the  qualities 
just  noted. 

The  refractive  index  is  the  ratio  between  the  sine  of  the  angle 
of  incidence  on  a  lens  surface  and  that  of  the  angle  of  refraction 
in  passing  the  surface.     Fig.  38  shows  the  relation  of  the  inci- 


FiG.   38. — The  Index  of  Refraction. 

dent  and  refracted  rays  in  passing  from  air  into  the  glass  lens 
surface  L,  and  the  sines  of  the  angles  which  determine  n,  the  con- 
ventional symbol  for  the  index  of  refraction.     Here  i  is  the  angle  of 

incidence  and  r  the  angle  of  refraction  i.e.  n  =  -7.     The  indices 

^  s 

of  refraction  are  usually  given  for  specific  colors  representing 
certain  lines  in  the  spectrum,  commonly  A'^,  the  potassium  line 
in  the  extreme  red,  C  the  red  line  due  to  hydrogen,  D  the  sodium 
line,  F  the  blue  hydrogen  line  and  G'  the  blue- violet  line  hydrogen 
line,  and  are  distinguished  as  Uc,  n^,  n/,  etc.  The  standard  disper- 
sion (dn)  for  visual  rays  is  given  as  between  C  and  F,  while  the 
standard  refractivity  is  taken  for  D,  in  the  bright  yellow  part 
of  the  spectrum.  (Note.  For  the  convenience  of  those  who  are 
rusty  on  their  trigonometry,  Fig.  39  shows  the  simpler  trigono- 
metric functions  of  an  angle.  Thus  the  sine  of  the  angle  A  is, 
numerically,  the  length  of  the  radius  divided  into  the  length  of 


64 


THE  TELESCOPE 


the  line  dropped  from  the  end  of  the  radius  to  the  horizontal 

base  line,  i.e.  jjlj  the  tangent  is  >^, ,  and  the  cosine  >jf . 

Ordinarily  the  index  of  refraction  of  the  crown  was  taken  as 
about  %,  that  of  the  flint  as  about  ^^.  As  time  has  gone  on 
and  especially  since  the  new  glasses  from  the  Jena  works  were 
introduced  about  35  years  ago,  one  cannot  define  crowns  and 
flints  in  any  such  simple  fashion,  for  there  are  crowns  of  high 
index  and  flints  of  low  dispersion. 


O  cosine  C       a 

Fig.  39.^ — The  Simple  Trigonometric  Functions  of  an  Angle. 

The  following  table  gives  the  optical  data  and  chemical  analy- 
ses of  a  few  typical  optical  glasses.  The  list  includes  common 
crowns  and  flints,  a  typical  baryta  crown  and  light  flint,  and  a 
telescope  crown  and  flint  for  the  better  achromatization  of  objec- 
tives, as  developed  at  the  Jena  works. 

The  thing  most  conspicuous  here  as  distinguishing  crowns  from 
flints  is  that  the  latter  have  greater  relative  dispersion  in  the 
blue,  the  former  in  the  red  end  of  the  spectrum,  as  shown  by  the 
bracketed  ratios.  This  as  we  shall  see  is  of  serious  consequence 
in  making  achromatic  objectives.  In  general,  too,  the  values  of 
V  for  flints  are  much  lower  than  for  crowns,  and  the  indices  of 
refraction  themselves  commonly  higher. 

As  we  have  just  seen,  glass  comes  to  the  optician  in  blocks 
or  discs,  for  miscellaneous  use  the  former,  three  or  four  inches 
square  and  an  inch  think,  more  or  less;  for  telescope  making  the 
latter.  The  discs  are  commonly  some  ten  percent  greater  in 
diameter  than  the  finished  objective  for  which  they  are  intended, 
and  in  thickness  from  3^^  to  3^f  o  the  diameter.  They  are  com- 
monly well  annealed  and  given  a  preliminary  polish  on  both  sides 
to  facilitate  close  inspection. 

The  first  step  toward  the  telescope  is  the  testing  of  these  discs 
of  glass,  first  for  the  presence  or  absence  of  striae  and  other 


OPTICAL  GLASS  AND  ITS  WORKING 


65 


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66 


THE  TELESCOPE 


imperfections;  second,  for  the  perfection  of  the  annealing.  The 
maker  has  usually  looked  out  for  all  the  grosser  imperfections 
before  the  discs  left  his  works,  but  a  much  closer  inspection  is 
needed  in  order  to  make  the  best  use  of  the  glass. 

Bad  strise  are  of  course  seen  easily,  as  they  would  be  in  a 
window  pane,  but  such  gross  imperfections  are  often  in  reality 
less  damaging  than  the  apparently  slighter  ones  which  must  be 
searched  for.     The  simplest  test  is  to  focus  a  good  telescope  on 


Fig.  40. — Testing  Glass  for  Striae. 

an  artificial  star,  remove  the  eyepiece  and  bring  the  eye  into  its 
place. 

When  the  eye  is  in  focus  the  whole  aperture  of  the  objective 
is  uniformly  filled  with  light,  and  if  the  disc  to  be  tested  be  placed 
in  front  of  it,  any  inequality  in  refraction  will  announce  itself  by 
an  inequality  of  illumination.  A  rough  judgment  as  to  the  seri- 
ousness of  the  defect  may  be  formed  from  the  area  affected  and 
the  amount  by  which  it  affects  the  local  intensity  of  illumination. 
Fig.  40  shows  the  arrangement  for  the  test,  A  being  the  eye,  B  the 


Fig.  41. — The  Mirror  Test  for  Stri«. 

objective  and  C  the  disc.  The  artificial  star  is  conveniently 
made  by  setting  a  black  bottle  in  the  sun  a  hundred  feet  or  so 
away  and,  getting  the  reflection  from  its  shoulder. 

A  somewhat  more  delicate  test,  very  commonly  used,  is  shown 
in  Fig.  41.  Here  A  is  a  truly  spherical  mirror  silvered  on  the 
front.  At  B  very  close  to  its  centre  of  curvature  is  placed  a  lamp 
with  a  screen  in  front  of  it  perforated  with  a  hole  ^^2  iiich  or  so 
in  diameter. 


OPTICAL  GLASS   AND  ITS  WORKING 


67 


The  rays  reflected  from  the  mirror  come  back  quite  exactly 
upon  themselves  and  when  the  eye  is  placed  at  C,  their  reflected 
focus,  the  whole  mirror  A  is  uniformly  lighted  just  as  the  lens 
was  in  Fig.  40,  with  the  incidental  advantage  that  it  is  much  easier 
and  cheaper  to  obtain  a  spherical  mirror  for  testing  a  sizeable 
disc  than  an  objective  of  similar  size  and  quality.  Now  placing 
the  disc  D  in  front  of  the  mirror,  the  light  passing  twice  through 
it  shows  up  the  slightest  stria  or  other  imperfection  as  a  streak 
or  spot  in  the  field.  Its  place  is  obvious  and  can  be  at  once 
marked  on  the  glass,  but  its  exact  position  in  the  substance  of  the 
disc  is  not  so  obvious. 

To  determine  this,  which  may  indicate  that  the  fault  can  be 
ground  out  in  shaping  the  lens,  a  modification  of  the  first  test 
serves  well,  as  indeed  it  does  for  the  general  examination  of  large 
discs.     Instead  of  using  a  distant  artificial  star  and  a  telescope, 


Fig.  42. — Locating  Striaj  in  the  Substance  of  a  Disc. 

one  uses  the  lamp  and  screen,  or  even  a  candle  flame  ten  feet  or 
more  away  and  a  condensing  lens  of  rather  short  focus,  which 
may  or  may  not  be  achromatic,  so  that  the  eye  will  get  into  its 
focus  conveniently  while  the  lens  is  held  in  the  hand.  Fig.  42 
shows  the  arrangement.  Here  A  is  the  eye,  B  the  condensing 
lens,  C  the  disc  and  D  the  source  of  light.  The  condensing  lens 
may  be  held  on  either  side  of  the  disc  as  convenience  suggests, 
and  either  disc  or  lens  may  be  moved.  The  operation  is  substan- 
tially the  examination  of  a  large  disc  piece-meal,  instead  of  all  at 
once  by  the  use  of  a  big  objective  or  mirror. 

Now  when  a  stria  has  been  noted  mark  its  location  as  to  the 
surface,  and,  moving  the  eye  a  little,  look  for  parallax  of  the  fault 
with  respect  to  the  surface  mark.  If  it  appears  to  shift  try  a 
mark  on  the  opposite  surface  in  the  same  way.  Comparison  of 
the  two  inspections  will  show  about  where  the  fault  lies  with 
respect  to  the  surfaces,  and  therefore  what  is  the  chance  of  work- 
ing it  out.  Sometimes  a  look  edgewise  of  the  disc  will  help  in 
the  diagnosis. 


68  THE  TELESCOPE 

Numerous  barely  detectable  striae  are  usually  worse  than  one 
or  two  conspicuous  ones,  for  the  latter  frequently  throw  the  light 
they  transmit  so  wide  of  the  focus  that  it  does  not  affect  the  image, 
which  could  be  greatly  damaged  by  slight  blurs  of  light  that  just 
miss  focus. 

Given  a  disc  that  passes  well  the  tests  for  striae  and  the  like 
the  next  step  is  to  examine  the  perfection  of  the  annealing,  which 
in  its  larger  aspect  is  revealed  by  an  examination  in  polarized  light. 

For  this  purpose  the  disc  is  set  up  against  a  frame  placed  on 
table  or  floor  with  a  good  exposure  to  skylight  behind  it,  and 


Fig.  43. — Testing  a  Disc  in  Polarized  Light. 

inclined  about  35°  from  the  vertical.  Behind  it  is  laid  a  flat 
shiny  surface  to  serve  as  polarizer.  Black  enamel  cloth  smoothly 
laid,  a  glass  plate  backed  with  black  paint,  or  even  a  smooth 
board  painted  with  asphalt  paint  will  answer  excellently.  Then 
holding  a  Nicol  prism  before  the  eye  and  looking  perpendicular 
to  the  face  of  the  disc,  rotate  the  prism  on  its  axis.  Fig.  43 
shows  the  arrangement,  A  being  the  eye,  B  the  Nicol,  C  the  disc, 
and  D  the  polarizer  behind  it. 

If  annealing  has  left  no  strain  the  only  effect  of  rotating  the 
Nicol  will  be  to  change  the  field  from  bright  to  dark  and  back 
again  as  if  the  disc  were  not  there.  Generally  a  pattern  in  the 
form  of  a  somewhat  hazy  Maltese  cross  will  appear,  with  its  arms 
crossing  the  disc,  growing  darker  and  lighter  alternately  as  the 
Nicol  is  turned. 


OPTICAL  GLASS  AND  ITS  WORKING  69 

If  the  cross  is  strongly  marked  but  symmetrical  and  well 
centered  the  annealing  is  fair — better  as  the  cross  is  fainter  and 
hazier — altogether  bad  if  colors  show  plainly  or  if  the  cross  is 
decentered  or  distorted.  The  test  is  extremely  sensitive,  so  that 
holding  a  finger  on  the  surface  of  the  disc  may  produce  local 
strain  that  will  show  as  a  faint  cloudy  spot. 

A  disc  free  of  striae  and  noticeable  annealing  strains  is  usually, 
but  not  invariably,  good,  for  too  frequent  reheating  in  the  mould- 
ing or  annealing  process  occasionally  leaves  the  glass  slightly 
altered,  the  effect  extending,  at  worst,  to  the  crystallization  or 
devitrification  to  which  reference  has  been  made. 

Given  a  good  pair  of  discs  the  first  step  towards  fashioning 
them  into  an  objective  is  roughing  to  the  approximate  form 
desired.  As  a  guide  to  the  shaping  of  the  necessary  curves, 
templets  must  be  made  from  the  designed  curves  of  the  objective 
as  precisely  as  possible.  These  are  laid  out  by  striking  the 
necessary  radii  with  beam  compass  or  pivoted  wire  and  scribing 
the  curve  on  thin  steel,  brass,  zinc  or  glass.  The  two  last  are 
the  easier  to  work  since  they  break  closely  to  form. 

From  these  templets  the  roughing  tools  are  turned  up,  com- 
monly from  cast  iron,  and  with  these,  supplied  with  carborundum 
or  even  sand,  and  water,  the  discs,  bearing  against  the  revolving 
tool,  are  ground  to  the  general  shape  required.  They  are  then 
secured  to  a  slowly  revolving  table,  bearing  edgewise  against  a 
revolving  grindstone,  and  ground  truly  circular  and  of  the  proper 
final  diameter. 

At  this  point  begins  the  really  careful  work  of  fine  grinding, 
which  must  bring  the  lens  very  close  to  its  exact  final  shape. 
Here  again  tools  of  cast  iron,  or  sometimes  brass,  are  used,  very 
precisely  brought  to  shape  according  to  the  templets.  They  are 
grooved  on  the  face  to  facilitate  the  even  distribution  of  the 
abrasive,  emery  or  fine  carborundum,  and  the  work  is  generally 
done  on  a  special  grinding  machine,  which  moves  the  tool  over 
the  firmly  supported  disc  in  a  complicated  series  of  strokes 
imitating  more  or  less  closely  the  strokes  found  to  be  most 
effective  in  hand  polishing. 

In  general  terms  the  operator  in  handwork  at  this  task  supports 
the  disc  on  a  firm  vertical  post,  by  cementing  it  to  a  suitable 
holder,  and  then  moves  the  tool  over  it  in  a  series  of  straight  or 
oval  strokes,  meanwhile  walking  around  the  post.  A  skilful 
operator  watches  the  progress  of  his  work,  varies  the  length  and 


70 


THE  TELESCOPE 


position  of  his  strokes  accordingly,  and,  despite  the  unavoidable 
wear  on  the  tool,  can  both  keep  its  figure  true  and  impart  a 
true  figure  to  the  glass. 

The  polishing  machine,  of  which  a  type  used  by  Dr.  Draper  is 
shown  in  Fig.  44,  produces  a  similar  motion,  the  disc  slowly 
revolving  and  the  rather  small  tool  moving  over  it  in  oval  strokes 
kept  off  the  center.  More  often  the  tool  is  of  approximately  the 
same  diameter  as  the  disc  under  it.  The  general  character  of 
the  motion  is  evident  from  the  construction.  The  disc  a  is 
chucked  by  c  c'  on  the  bed,  turned  by  the  post  d  and  worm  wheel 
e.  This  is  operated  from  the  pulleys,  i,  g,  which  drive  through  k. 
the  crank  m,  adjustable  in  throw  by  the  nuts  n,n',  and  in  position 
of  tool  by  the  clamps  r,  r.     The  motion  may  be  considerably 


Fig.  44. — Dr.  Draper's  Polishing  Machine. 


varied  by" adjustment  of  the  machine,  always  keeping  the  stroke 
from  repeating  on  the  same  part  of  the  disc,  by  making  the  period 
of  the  revolution  and  of  the  stroke  incommensurable  so  far  as 
may  be.  Even  in  spectacle  grinding  machines  the  stroke  may 
repeat  only  once  in  hundreds  of  times,  and  even  this  frequency 
in  a  big  objective  would,  if  followed  in  the  polishing,  leave  tool 
marks  which  could  be  detected  in  the  final  testing. 

In  the  fine  grinding,  especially  near  the  end  of  the  process,  the 
templets  do  not  give  sufficient  precision  in  testing  the  curves,  and 
recourse  is  had  to  the  spherometer,  by  which  measurements 
down  to  about  Mooooo  inch  can  be  consistently  made. 

The  next  stage  of  operations  is  polishing,  which  transforms  the 
grey  translucency  of  the  fine  ground  lens  into  the  clear  and 
brilliant  surface  which  at  last  permits  rigorous  optical  tests  to  be 
used  for  the  final  finish  of  the  lens.     This  polishing  is  done  gen- 


OPTICAL  GLASS  AND  ITS  WORKING  71 

erally  on  the  fine  grinding  machine  but  with  a  very  different  tool 
and  with  rouge  of  the  utmost  fineness. 

The  pohshing  tool  is  in  any  case  ground  true  and  is  then  faced 
with  a  somewhat  yielding  material  to  carry  the  charge  of  rouge. 
Cheap  lenses  are  commonly  worked  on  a  cloth  polisher,  a  texture 
similar  to  billard  cloth  being  suitable,  or  sometimes  on  paper 
worked  dry. 

With  care  either  may  produce  a  fairly  good  surface,  with, 
however,  a  tendency  to  polish  out  the  minute  hollows  left  by 
grinding  rather  than  to  cut  a  true  surface  clear  down  to  their 
bottoms.  Hence  cloth  or  paper  is  likely  to  leave  microscopic 
inequalities  apparently  polished,  and  this  may  be  sufficient  to 
scatter  over  the  field  a  very  perceptible  amount  of  light  which 
should  go  to  forming  the  image.  All  first  class  objectives  and 
mirrors  are  in  fact  polished  on  optician's  pitch.  This  is  not  the 
ordinary  pitch  of  commerce  but  a  substance  of  various  composi- 
tion, sometimes  an  asphaltic  compound,  again  on  a  base  of  tar, 
or  of  resin  brought  to  the  right  consistency  by  turpentine. 

Whatever  the  exact  composition,  the  fundamental  property 
is  that  the  material,  apparently  fairly  hard  and  even  brittle  when 
cold,  is  actually  somewhat  plastic  to  continued  pressure.  Sealing 
wax  has  something  of  this  quality,  for  a  stick  which  may  readily 
be  broken  will  yet  bend  under  its  own  weight  if  supported  at  the 
ends. 

If  the  fine  grinding  process  has  been  properly  carried  out  the 
lens  has  received  its  correct  form  as  nearly  as  gauges  and  the 
spherometer  can  determine  it.  The  next  step  is  to  polish  the  sur- 
face as  brilliantly  and  evenly  as  possible.  To  this  end  advantage 
is  taken  of  the  plastic  quality  already  mentioned,  that  the  glass 
may  form  its  own  tool. 

The  base  of  the  tool  may  be  anything  convenient,  metal,  glass 
or  even  wood.  Its  working  surface  is  made  as  nearly  of  the  right 
curvature  as  practicable  and  it  is  then  coated  with  warm  pitch 
to  a  thickness  of  an  eighth  of  an  inch  more  or  less,  either  continu- 
ously or  in  squares,  and  while  still  slightly  warm  the  tool  is  placed 
against  the  fine  ground  disc,  the  exact  shape  of  which  it  takes. 

When  cold  the  pitch  surface  can  easily  be  cut  out  into  squares 
or  symmetricallj'^  pitted  with  a  suitable  tool,  at  once  facilitating 
the  distribution  of  the  rouge  and  water  that  serves  for  polishing, 
and  permitting  delicate  adjustment  of  the  working  curvature  in 
a  way  about  to  be  described. 


72 


THE  TELESCOPE 


Fig.  45  shows  the  squared  surface  of  the  tool  as  it  would  be 
used  for  polishing  a  plane  or  very  slightly  convex  or  concave 
surface.  Supplied  with  the  thin  abrasive  paste,  it  is  allowed 
to  settle,  cold,  into  its  final  contact  with  the  glass,  and  then  the 
process  of  polishing  by  hand  or  machine  is  started. 

The  action  of  the  tool  must  be  uniform  to  avoid  changing 
the  shape  of  the  lens.  It  can  be  regulated  as  it  was  in  the  grind- 
ing, by  varying  the  length  and  character  of  the  stroke,  but  even 
more  delicately  by  varying  the  extent  of  surface  covered  by  the 
pitch  actually  working  on  the  glass. 

This  is  done  by  channeling  or  boring  away  pitch  near  the  rim 
or   center   of   the    tool   as  the  case  may  be.     Fig.  46  shows  a 


Fig.  45.— Tool  for  Flat  Surface. 


Fig.   46. — Tool  for  Concave  Surface. 


tool  which  has  been  thus  treated  so  that  the  squares  are  progres- 
sively smaller  near  the  periphery.  Such  a  spacing  tends  to 
produce  a  concave  surface  from  a  flat  tool  or  to  increase  the 
concavity  from  a  curved  one.  Trimming  down  the  squares 
towards  the  centre  produces  the  opposite  result. 

Broadly,  the  principle  is  that  the  tool  cuts  the  more  in  the 
areas  where  the  contact  surfaces  are  the  greater.  This  is  not 
wholly  by  reason  of  greater  abrading  surface,  but  also  because 
where  the  contact  is  greater  in  area  the  pitch  settles  less,  from 
the  diminished  pressure,  thus  increasing  the  effective  contact. 

Clearly  the  effect  of  trimming  away  is  correlated  with  the 
form  and  length  of  stroke,  and  the  temper  of  the  pitch,  and  in 
fact  it  requires  the  wisdom  of  the  serpent  to  combine  these 
various  factors  so  as  to  produce  the  perfectly  uniform  and  regular 
action  required  in  polishing.     Now  and  then,  at  brief  intervals, 


OPTICAL  GLASS  AND  ITS  WORKING  73 

the  operation  is  stopped  to  supply  rouge  and  to  avoid  changing 
the  conditions  by  the  heat  of  friction.  Especially  must  heating 
be  looked  out  for  in  hand  polishing  of  lenses  which  is  often  done 
with  the  glass  uppermost  for  easier  inspection  of  the  work. 

Polishing,  if  the  fine  grinding  has  been  judiciously  done  is, 
for  moderate  sized  surfaces,  a  matter  of  only  a  few  hours.  It 
proceeds  quite  slowly  at  first  while  the  hills  are  being  ground  down 
and  then  rather  suddenly  comes  up  brilliantly  as  the  polisher 
reaches  the  bottoms  of  the  valleys.  Large  lenses  and  mirrors  may 
require  many  days. 

Now  begins  the  final  and  extraordinarily  delicate  process  of 
figuring.  The  lens  or  mirror  has  its  appointed  form  as  nearly 
as  the  most  precise  mechanical  methods  can  tell — say  down  to  one 
or  two  hundred-thousandths  of  an  inch.  From  the  optical 
standpoint  the  result  may  be  thoroughly  bad,  for  an  error  of  a 
few  millionths  of  an  inch  may  be  serious  in  the  final  performance. 

The  periphery  may  be  by  such  an  amount  longer  or  shorter  in 
radius  than  it  should  be,  or  there  may  be  an  intermediate  zone 
that  has  gone  astray.  In  case  of  a  mirror  the  original  polishing  is 
generally  intended  to  leave  a  spherical  surface  which  must  be  con- 
verted into  a  paraboloidal  one  by  a  change  in  curvature  totalling 
only  a  few  hundred-thousandths  of  an  inch  and  seriously  affec- 
ted by  much  smaller  variations. 

The  figuring  is  done  in  a  fashion  very  similar  to  the  polishing. 
The  first  step  is  to  find  out  by  optical  tests  such  as  are  described 
in  Chapter  IX  the  location  of  the  errors  existing  after  the  polish- 
ing, and  once  found,  they  must  be  eliminated  by  patient  and 
cautious  work  on  the  surface. 

Every  optical  expert  has  his  own  favorite  methods  of  working 
out  the  figure.  If  there  is  a  hollow  zone  the  whole  surface  must 
be  worked  down  to  its  level  by  repolishing;  if,  on  the  other  hand, 
there  is  an  annular  hump,  one  may  repolish  with  stroke  and  tool- 
face  adapted  to  cut  it  down,  or  one  may  cautiously  polish  it  out 
until  it  merges  with  the  general  level. 

Polishing  is  commonly  done  with  tools  of  approximately  the 
size  of  the  work,  but  in  figuring  there  is  great  difference  of  practice, 
some  expert  workers  depending  entirely  on  manipulation  of  a  full 
sized  tool,  others  working  locally  with  small  polishers,  even  with 
the  ball  of  the  thumb,  in  removing  slight  aberrations.  In  small 
work  where  the  glass  can  be  depended  on  for  homogeneity  and 
the  tools  are  easily  kept  true  the  former  method  is  the  usual 


74  THE  rpJLESCOPE 

one,  but  in  big  objectives  the  latter  is  often  easier  and  may 
successfully  reach  faults  otherwise  very  difficult  to  eliminate. 

Among  well  known  makers  of  telescopes  the  Clarks  and  their 
equally  skilled  successors  the  Lundins,  father  and  son,  developed 
the  art  of  local  retouching  to  a  point  little  short  of  wizardry;  the 
late  Dr.  Brashear  depended  almost  entirely  on  the  adroitly 
used  polishing  machine;  Sir  Howard  Grubb  uses  local  correction 
in  certain  cases,  and  in  general  the  cautiously  modified  polisher; 
while  some  of  the  Continental  experts  are  reported  to  have 
developed  the  local  method  very  thoroughly. 

The  truth  probably  is  that  the  particular  error  in  hand  should 
determine  the  method  of  attack  and  that  its  success  depends 
entirely  on  the  skill  of  the  operator.  As  to  the  perfection  of  the 
objectives  figured  in  either  way,  no  systematic  difference  due  to 
the  method  of  figuring  can  be  detected  by  the  most  delicate  tests. 

In  any  case  the  figuring  operation  is  long  and  tedious,  especially 
in  large  work  where  problems  of  supporting  to  avoid  flexure  arise, 
where  temperature  effects  on  tool  and  glass  involve  long  delays 
between  tests  and  correction,  and  where  in  the  last  resort  non- 
spherical  surfaces  must  often  be  resorted  to  in  bringing  the  image 
to  its  final  perfection. 

The  final  test  of  goodness  is  performance,  a  clean  round  image 
without  a  trace  of  spherical  or  zonal  aberration  and  the  color 
correction  the  best  the  glasses  will  allow.  Constant  and  rigorous 
testing  must  be  applied  all  through  the  process  of  figuring,  and 
the  result  seems  to  depend  on  a  combination  of  experience,  intui- 
tion  and   tactual  expertness  rarely  united  in  any  one  person. 

Sir  Howard  Grubb,  in  a  paper  to  be  commended  to  anyone 
interested  in  objectives,  once  forcibly  said:  "I  may  safely  say 
that  I  have  never  finished  any  objective  over  10  inches  diameter, 
in  the  working  of  which  I  did  not  meet  with  some  new  experience, 
some  new  set  of  conditions  which  I  had  not  met  before,  and  which 
had  then  to  be  met  by  special  and  newly  devised  arrangements." 

The  making  of  reflecting  telescopes  is  not  much  easier  since 
although  only  one  surface  has  to  be  worked,  that  one  has  to  be 
figured  with  extraordinary  care,  flexure  has  to  be  guarded  against 
at  every  stage  of  the  working,  and  afterwards,  temperature 
change  is  a  busy  foe,  while  testing  for  correct  figure,  the  surface 
being  non-spherical,  is  considerably  more  troublesome. 

An  expert  can  make  a  good  mirror  with  far  less  actual  labor 
than  an  objective  of  similar  aperture,  but  when  one  reads  Dr. 


OPTICAL  GLASS  AND  ITS  WORKING  75 

Henry  Draper's  statement  that  in  spite  of  knowing  at  first  hand 
the  methods  and  grinding  machines  of  Lord  Rosse  and  Mr.  Lassell, 
he  ground  over  a  hundred  mirrors,  and  spent  three  years  of  time, 
before  he  could  get  a  correct  figure  with  reasonable  facility,  one 
certainly  gains  a  high  respect  for  the  skill  acquired. 

This  chapter  is  necessarily  sketchy  and  not  in  the  least  intended 
to  give  the  reader  a  complete  account  of  technical  glass  manufac- 
ture, far  less  of  the  intricate  and  almost  incommunicable  art 
of  making  objectives  and  mirrors.  It  may  however  lead  to  a 
better  understanding  of  the  difference  between  the  optical 
glass  industry  and  the  fabrication  of  commercial  glass,  and  lead 
the  reader  to  a  fuller  realization  of  how  fine  a  work  of  art  is  a 
finished  objective  or  mirror  as  compared  with  the  crude  efforts 
of  the  early  makers  or  the  hasty  bungling  of  too  many  of  their 
successors. 

For  further  details  on  making,  properties  and  working  of 
optical  glass  see: 

Hovestadt:  "Jenaer  Glas." 

Rosenhain:  "Glass  Manufacture." 

Sir  Howard  Grubb:  "Telescopic  Objectives  and  Mirrors:  Their  Prepara- 
tion and  Testing.     Nature  34,  85. 

Dr.  Henry  Draper:  "On  the  Construction  of  a  Silvered  Glass  Telescope." 
(Smithsonian  Contributions  to  Knowledge,  Vol.  34.) 

G.  W.  Ritchey:  On  the  Modern  Reflecting  Telescope  and  the  Making  and 
Testing  of  Optical  Mirrors.  (Smithsonian  Contributions  to  Know- 
ledge, Vol.  34.) 

Lord  Rayleigh  Polishing  of  Glass  Surfaces.  (Proc.  Opt.  Convention,  1905, 
p.  73.) 

Bottone:  "Lens  Making  for  Amateurs." 


CHAPTER  IV 

THE  PROPERTIES  OF  OBJECTIVES  AND  MIRRORS 

The  path  of  the  rays  through  an  ordinary  telescope  has  been 
shown  in  Fig.  5.  In  principle  all  the  rays  from  a  point  in  the 
distant  object  should  unite  precisely  in  a  corresponding  point 
in  the  image  which  is  viewed  by  the  eyepiece.  Practically  it 
takes  very  careful  design  and  construction  of  the  objective  to 
make  them  meet  in  such  orderly  fashion  even  over  an  angular 
space  of  a  single  degree,  and  the  wider  the  view  required  the  more 
difficult  the  construction.  We  have  spoken  in  the  account  of  the 
early  workers  of  their  struggles  to  avoid  chromatic  and  spherical 
aberrations,  and  it  is  chiefly  these  that  still,  in  less  measure, 
worry  their  successors. 


Fig.  47. — Chromatic  Aberration  of  Convex  Lens. 

The  first  named  is  due  to  the  fact  that  a  prism  does  not  bend 
light  of  all  colors  equally,  but  spreads  them  out  into  a  spectrum ; 
red  refracted  the  least,  violet  the  most.  Since  a  lens  may  be 
regarded  as  an  assemblage  of  prisms,  of  small  angle  near  the  cen- 
tre and  greater  near  the  edge,  it  must  on  the  whole  and  all  over 
bend  the  blue  and  violet  rays  to  meet  on  the  axis  nearer  the  rear 
surface  than  the  corresponding  red  rays,  as  shown  in  Fig.  47. 
Here  the  incident  ray  a  is  split  up  by  the  prismatic  effect  of  the 
lens,  the  red  coming  to  a  focus  at  r,  the  violet  at  v. 

One  can  readily  see  this  chromatic  aberration  by  covering  up 
most  of  a  common  reading  glass  with  his  hand  and  looking  through 
the  edge  portion  at  a  bright  light,  which  will  be  spread  out  into 
a  colored  band. 

76 


THE  PROPERTIES  OF  OBJECTIVES  AND  MIRRORS        77 

If  the  lens  is  concave  the  violet  rays  will  still  be  the  more 
bent,  but  now  outwards,  as  shown  in  Fig.  48.  The  incident  ray 
a'  is  split  up  and  the  violet  is  bent  toward  v,  proceeding  as  if 
coming  straight  from  a  virtual  focus  v'  in  front  of  the  lens,  and 
nearer  it  than  the  corresponding  red  focus  r'.  Evidently  if  we 
could  combine  a  convex  lens,  bending  the  violet  inward  too  much, 
with  a  concave  one,  bending  it  outward  too  much,  the  two 
opposite  variations  might  compensate  each  other  so  that  red  and 
violet  would  come  to  the  same  focus — which  is  the  principle  of 
the  achromatic  objective. 

If  the  refractive  powers  of  the  lenses  were  exactly  proportional 
to  their  dispersive  powers,  as  Newton  erroneously  thought,  it  is 
evident  that  the  concave  lens  would  pitch  all  the  rays  outwards 


Fig.  48. — Chromatic  Aberration  of  Concave  Lens. 

to  an  amount  which  would  annul  both  the  chromatic  variation 
and  the  total  refraction  of  the  convex  lens,  leaving  the  pair 
without  power  to  bring  anything  to  a  focus.  Fortunately  flint 
glass  as  compared  with  crown  glass  has  nearly  double  the  dis- 
persion between  red  and  violet,  and  only  about  20%  greater 
refractive  power  for  the  intermediate  yellow  ray. 

Hence,  the  prismatic  dispersive  effect  being  proportional  to 
the  total  curvature  of  the  lens,  the  chromatic  aberration  of  a 
crown  glass  lens  will  be  cured  by  a  concave  flint  lens  of  about  half 
the  total  curvature,  and,  the  refractions  being  about  as  5  to  6,  of 
%  the  total  power. 

Since  the  ''power"  of  any  lens  is  the  reciprocal  of  its  focal 
length,  a  crown  glass  convex  lens  of  focal  length  3,  and  a  concave 
flint  lens  of  focal  length  5  (negative)  will  form  an  approximately 
achromatic  combination.  The  power  of  the  combination  will  be 
the  algebraic  sum  of  the  powers  of  the  components  so  that  the 


78  THE  TELESCOPE 

focal  length  of  the  pair  will  be  about  %  that  of  the  crown  lens 
with  which  we  started. 

To  be  more  precise  the  condition  of  achromatism  is 

Sp5n  +  Sp'5n'  =  0 

where  p  is  the  reciprocal  of  a  radius  and  5n,  or  6n',  is  the  differ- 
ence in  refractive  index  between  the  rays  chosen  to  be  brought  to 
exact  focus  together,  as  the  red  and  the  blue  or  violet. 

This  conventional  equation  simply  states  that  the  sum  of  the 
reciprocals  of  the  radii  of  the  crown  lens  multiplied  by  the  dis- 
persion of  the  crown,  must  equal  the  corresponding  quantity  for 
the  flint  lens  if  the  two  total  dispersions  are  to  annul  each  other, 
leaving  the  combination  achromatic.  Whatever  glass  is  used 
the  power  of  a  lens  made  of  it  is 

P(=^)  ^  Sp(n-  1) 

so  that  it  will  be  seen  that,  other  things  being  equal,  a  glass  of 
high  index  of  refraction  tends  to  give  moderate  curves  in  an  objec- 
tive. Also,  referring  to  the  condition  of  achromatism,  the  greater 
the  difference  in  dispersion  between  the  two  glasses  the  less 
curvatures  will  be  required  for  a  given  focal  length,  a  condition 
advantageous  for  various  reasons. 

The  determination  of  achromatism  for  any  pair  of  glasses  and 
focal  length  is  greatly  facilitated  by  employing  the  auxiliary 
quantity  v  which  is  tabulated  in  all  lists  of  optical  glass  as  a 
short  cut  to  a  somewhat  less  manageable  algebraic  expression. 
Using  this  we  can  figure  achromatism  for  unity  focal  length  at 
once, 

V  —  V  V  —  V  on 

being  the  powers  of  the  leading  and  following  lenses  respectively. 
The  combined  lens  will  bring  the  rays  of  the  two  chosen  colors,  as 
red  and  blue,  to  focus  at  the  same  point  on  the  axis.  It  does  not 
necessarily  give  to  the  red  and  blue  images  of  an  object  the  same 
exact  size.  Failure  in  this  respect  is  known  as  chromatic  difference 
of  magnification,  but  the  fault  is  small  and  may  generally  be 
neglected  in  telescope  objectives. 

We  have  now  seen  how  an  objective  may  be  made  achromatic 
and  of  determinate  focal  length,  but  the  solution  is  in  terms  of  the 
sums  of  the  respective  curvatures  of  the  crown  and  flint  lenses, 


THE  PROPERTIES  OF  OBJECTIVES  AND  MIRRORS        79 

and  gives  no  information  about  the  radii  of  tiie  individual  sur- 
faces. The  relation  between  these  is  all-important  in  the  final 
performance. 

For  in  a  convex  lens  with  spherical  surfaces  the  rays  striking 
near  the  edge,  of  whatever  color,  are  pitched  inwards  too  much 


Fig.  49. — Spherical  Aberration  of 


compared  with  rays  striking  the  more  moderate  curvatures  near 
the  axis,  as  shown  in  Fig.  49.  The  ray  a'  h'  thus  comes  to  a  focus 
shorter  than  the  ray  a  h. 

This  constitutes  the  fault  of  spherical  aberration,  which  the  old 
astronomers,  following  the  suggestions  of  Descartes,  tried  in- 
effectually to  cure  by  forming  lenses  with  non-spherical  surfaces. 


Fig.  50. — Spherical  Aberration  of  Concave  Lens. 

Fig.  50  suggests  the  remedy,  for  the  outer  ray  a"  is  pitched  out 
toward  h"  as  if  it  came  from  a  focal  point  c",  while  the  ray  nearer 
the  center  a'"  is  much  less  bent  toward  b'"  as  if  it  came  from  c'". 
The  spherical  aberrations  of  a  concave  lens  therefore,  being  oppo- 
site to  those  of  a  convex  lens,  the  two  must,  at  least  to  a  certain 
extent,  compensate  each  other  as  when  combined  in  an 
achromatic  objective. 


80 


THE  TELESCOPE 


So  in  fact  they  do,  and,  if  the  curves  that  go  to  make  up  the 
total  curvatures  of  the  two  are  properly  chosen,  the  total  spher- 
ical aberration  can  be  made  negligibly  small,  at  least  on  and  near 
the  axis.  Taking  into  account  this  condition,  therefore,  at  once 
gives  us  a  clue  to  the  distribution  of  the  total  curvatures  and 
hence  to  the  radii  of  the  two  lenses.  Spherical  aberration, 
however,  involves  not  only  the  curvatures  but  the  indices  of 
refraction,  so  that  exact  correction  depends  in  part  on  the  choice 
of  glasses  wherewith  to  obtain  achromatization. 

In  amount  spherical  aberration  varies  with  the  square  of  the 

aperture  and  inversely  with  the  cube  of  the  focal  length  i.e. 

a^ 
with  Yz      It  is  reckoned  as   +  when,  as  in  Fig.  49,  the  rim  rays 

come   to  the  shorter  focus,  as  — ,  when  they  come  to  the  longer 
focus. 

In  any  event,  since  the  spherical  aberration  of  a  lens  may  be 
varied  in  above  the  ratio  of  4:1,  for  the  same  total  power,  merely 
by  changing  the  ratio  of  the  radii,  it  is  evident  that  the  two  lenses 
being  fairly  correct  in  total  curvature  might  be  given  considerable 
variations  in  curvature  and  still  mutually  annul  the  axial  spher- 
ical aberration. 

Such  is  in  fact  the  case,  so  that  to  get  determinate  forms  for 
the  lenses  one  must  introduce  some  further  condition  or  make 
some  assumption  that  will  pin  down  the  separate  curvatures  to 

some  definite  relations.  The  require- 
ment may  be  entirely  arbitrary,  but 
in  working  out  the  theory  of  objec- 
tives has  usually  been  chosen  to  give 
the  lens  some  real  or  hypothetical 
additional  advantage. 

The  commonest  arbitrary  require- 
ment is  that  the  crown  glass  lens 
shall  be  equi convex,  merely  to  avoid 
making  an  extra  tool.  This  fixes  one 
pair  of  radii,  and  the  flint  lens  is  then  given  the  required  com- 
pensating aberration  choosing  the  easiest  form  to  make.  This 
results  in  the  objective  of  Fig.  51. 

Probably  nine  tenths  of  all  objectives  are  of  this  general  form, 
equiconvex  crown  and  nearly  or  quite  plano-concave  flint.  The 
inside  radii  may  be  the  same,  in  which  case  the  lenses  should  be 
cemented,  or  they  may  differ  slightly  in  either  direction  as  a.  Fig.  51 


Fig.  51. — Objectives  with  Equi- 
convex Crown. 


THE  PROPERTIES  OF  OBJECTIVES  AND  MIRRORS        81 

with  the  front  of  the  flint  less  curved  than  the  rear  of  the  crown, 
and  h  where  the  flint  has  the  sharper  curve.  The  resulting 
lens  if  ordinary  glasses  are  chosen  gives  excellent  correction  of  the 
spherical  aberration  on  the  axis,  but  not  much  away  from  it, 
yielding  a  rather  narrow  sharp  field.  Only  a  few  exceptional 
combinations  of  glasses  relieve  this  situation  materially. 

The  identity  of  the  inner  radii  so  that  the  surfaces  can  be 
cemented  is  known  historically  as  Clairault's  condition,  and  since 
it  fixes  two  curvatures  at  identity  somewhat  limits  the  choice  of 
glasses,  while  to  get  proper  corrections  demands  quite  wide 
variations  in  the  contact  radii  for  comparatively  small  variations 
in  the  optical  constants  of  the  glass. 

When  two  adjacent  curves  are  identical  they  should  be 
cemented,  otherwise  rays  reflected  from  say  the  third  surface  of 
Fig.  51  will  be  reflected  again  from  the  second  surface,  and  passing 
through  the  rear  lens  in  almost  the  path  of  the  original  ray  will 
come  to  nearly  the  same  focus,  producing  a  troublesome  "ghost." 
Hence  the  curvatures  of  the  second  and  third  surfaces  when  not 
cemented  are  varied  one  way  or  the  other  by  two  or  three  per 
cent,  enough  to  throw  the  twice  reflected  rays  far  out  of  focus. 

In  this  case,  as  in  most  others,  the  analytical  expression  for  the 
fundamental  curvature  to  be  determined  turns  up  in  the  form  of 
a  quadratic  equation,  so  that  the 
result  takes  the  form  a  ±  s/b  and 
there  are  two  sets  of  radii  that 
meet  the  requirements.  Of  these 
the  one  presenting  the  gentler 
curves  is  ordinarily  chosen.  Fig. 
52  a  and  c  shows  the  two  cemented 

forms,     thus     related,     for    a     com-  ^^^  52.-Allied  Forms  of  Cemented 

mon     pair    of     crown     and     flint  Objectives. 

glasses,  both  cleanly  corrected  for  chromatic  and  axial  spherical 

aberration. 

Nearly  a  century  ago  Sir  John  Herschel  proposed  another 
defining  condition,  that  the  spherical  aberration  should  be  removed 
both  for  parallel  incident  rays  and  for  those  proceeding  from  a 
nearer  point  on  the  axis,  say  ten  or  more  times  the  focal  length  in 
front  of  the  objective.  This  condition  had  little  practical  value 
in  itself,  and  its  chief  merit  was  that  it  approximated  one  that 
became  of  real  importance  if  the  second  point  were  taken  far 
enough  away. 

6 


82  THE  TELESCOPE 

A  little  later  Gauss  suggested  that  the  spherical  aberration 
should  be  annulled  for  two  different  colors,  much  as  the  chromatic 
aberration  is  treated.  And,  being  a  mathematical  wizard,  he 
succeeded  in  working  out  the  very  intricate  theory,  which  resulted 
in  an  objective  approximately  of  the  form  shown  in  Fig.  53. 

It  does  not  give  a  wide  field  but  is  valuable  for  spectroscopic 
work,  where  keen  definition  in  all  colors  is  essen- 
tial. Troublesome  to  compute,  and  difficult  to 
mount  and  center,  the  type  has  not  been  much 
used,  though  there  are  fine  examples  of  about 
93-^  inches  aperture  at  Princeton,  Utrecht,  and 
Copenhagen,  and  a  few  smaller  ones  elsewhere, 
chiefly  for  spectroscopic  use. 

jPj^  53 Gaus-        ^^   ^^^   Fraunhofer  who  found    and  applied 

sian Objective,  the  determining  condition  of  the  highest  practical 
value  for  most  purposes.  This  condition  was  absence  of  coma, 
the  comet  shaped  blur  generally  seen  in  the  outer  portions  of  a 
wide  field. 

It  is  due  to  the  fact  that  parallel  oblique  rays  passing  through 
the  opposite  rims  of  the  lens  and  through  points  near  its  center 
do  not  commonly  come  to  the  same  focus,  and  it  practically 
is  akin  to  a  spherical  aberration  for  oblique  rays  which  greatly 
reduces  the  extent  of  the  sharp  field.  It  is  reckoned  +  when 
the  blur  points  outwards,  — when  it  points  inwards,  and  is  directly 
proportional  to  the  tangent  of  the  obliquity  and  the  square  of  the 
aperture,  and  inversely  to  the  square  of  the  focal  length  i.e.  it 

.  ,   a^  tanu 
varies  with  — y^ 

Just  how  Fraunhofer  solved  the  problem  is  quite  unknown,  but 
solve  it  he  did,  and  very  completely,  as  he  indicates  in  one  of  his 
later  papers  in  which  he  speaks  of  his  objective  as  reducing  all  the 
aberrations  to  a  minimum,  and  as  Seidel  proved  30  years  later  in 
the  analysis  of  one  of  Fraunhofer's  objectives.  Very  probably 
he  worked  by  tracing  axial  and  oblique  rays  through  the  objective 
form  by  trigonometrical  computation,  thus  finding  his  way  to  a 
standard  form  for  the  glasses  he  used.^ 

1  More  recently  his  condition  proves  to  be  quite  the  exact  equivalent  of 
Abbe's  sine  condition  which  states  that  the  sine  of  the  angle  made  with  the 
optical  axis  by  a  ray  entering  the  objective  from  a  given  axial  point  shall 
bear  a  uniform  ratio  to  the  sine  of  the  corresponding  angle  of  emergence, 
whatever  the  point  of  incidence.  For  parallel  rays  along  the  axis  this 
reduces  to  the  requirement  that  the  sines  of  the  angles  of  emergence  shall 
be  proportional  to  the  respective  distances  of  the  incident  rays  from  the  axis. 


THE  PROPERTIES  OF  OBJECTIVES  AND  MIRRORS        83 


Fig.  54.— The  Fraun- 
hofer  Types. 


Fraunhofer's  objective,  of  which  Fig.  54a  is  an  example 
worked  by  modern  formulae  for  the  sine  condition,  gives  very  exact 
corrections  over  a  field  of  2°-3°  when  the  glasses  are  suitably 
chosen  and  hence  is  invaluable  for  any  work  requiring  a  wide 
angle  of  view. 

With  certain  combinations  of  glasses  the  coma-free  condition 
may  be  combined  successfully  with  Clairault's, 
although  ordinarily  the  coma-free  form  falls 
between  the  two  forms  clear  of  spherical  aber- 
ration, as  in  Fig.  52,  b,  which  has  its  oblique 
rays  well  compensated  but  retains  serious  axial 
faults. 

Fraunhofer's  objective  has  for  all  advan- 
tageous combinations  of  glasses  the  front 
radius  of  the  flint  longer  than  the  rear  radius 
of  the  crown  hence  the  two  must  be  separated 
by  spacers  at  the  edge,  which  in  small  lenses  in  simple  cells  is 
slightly  inconvenient.  However,  the  common  attempt  to  sim- 
plify mounting  by  making  the  front  flint  radius  the  shorter 
almost  invariably  violates  the  sine  condition  and  reduces  the 
sharp  field,  fortunately  not  a  very  serious  matter  for  most  astro- 
nomical work. 

The  only  material  objection  to  the  Fraunhofer  type  is  the 
strong  curvature  of  the  rear  radius  of  the  crown  which  gives  a 
form  somewhat  susceptible  to  flexure  in  large  objectives.  This  is 
met  in  the  flint-ahead  form,  developed  essentially  by  Steinheil, 
and  used  in  most  of  the  objectives  of  his  famous  firm.  Fig.  546 
shows  the  flint-ahead  objective  corresponding  to 
Fig.  54a.  Obviously  its  curves  are  mechanically 
rather  resistant  to  flexure.^ 

Mechanical  considerations  are  not  unimportant  in 
large  objectives,   and   Fig.  55,  a  highly  useful  form 
introduced   by  the  Clarks  and  used  in  recent  years 
for  all  their  big  lenses,  is  a  case  in  point.      Here  there 
^^°- ^^.-^  is  an  interval  of  about  the  proportion  shown  between 
tive.  the  crown  and  flmt  components. 

This  secures  effective  ventilation  allowing  the  lenses  to  come 
quickly  to   their   steady  temperature,    and   enables   the   inner 

'  It  is  interesting  to  note  that  in  computing  Fig.  54a  for  the  sine  condi- 
tion, the  other  root  of  the  quadratic  gave  roughly  the  Gaussian  form  of 
Fig.  53. 


_  ^LtA^V^A  JTO 


84  THE  TELESCOPE 

surfaces  to  be  cleaned  readily  and  freed  of  moisture.  Optically 
it  lessens  the  deviation  from  the  sine  condition  otherwise  prac- 
tically inseparable  from  the  equiconvex  crown,  reduces  the 
chromatic  difference  of  spherical  aberration,  and  gives  an  easy 
way  of  controlling  the  color  correction  by  slightly  varying  the 
separation  of  the  lenses. 

One  further  special  case  is  worth  noting,  that  of  annulling  the 
spherical  aberration  for  rays  passing  through  the  lens  in  both 
directions.  By  proper  choice  of  glass  and  curvatures  this  can  be 
accomplished  to  a  close  approximation  and  the 
resulting  form  is  shown  in  Fig.  56.  The  front  of  the 
crown  is  notably  flat  and  the  rear  of  the  flint  con- 
spicuously curved,  the  shape  in  fact  being  inter- 
mediate between  Figs.  526  and  52c.  The  type  is 
useful  in  reading  telescopes  and  the  like,  and  for 
some  spectroscopic  applications. 
Fig.  56. —  There  are  two  well  known  forms  of  aberration  not 
^°"®°J:®*^ '"^  yet  considered;  astigmatism  and  curvature  of  field. 
tions.  The  former  is  due  to  the  fact  that  when  the  path  of 

the  rays  is  away  from  the  axis,  as  from  an  extended  object, 
those  coming  from  a  line  radial  to  the  axis,  and  those  from  a  line 
tangent  to  a  circle  about  the  axis,  do  not  come  to  the  same 
focus.  The  net  result  is  that  the  axial  and  tangential  elements 
are  brought  to  focus  in  two  coaxial  surfaces  touching  at  the  axis 
and  departing  more  and  more  widely  from  each  other  as  they 
depart  from  it.  Both  surfaces  have  considerable  curvature,  that 
for  tangential  lines  being  the  sharper. 

It  is  possible  by  suitable  choice  of  glasses  and  their  curvatures 
to  bring  both  image  surfaces  together  into  an  approximate  plane 
for  a  moderate  angular  space  about  the  axis  without  seriously 
damaging  the  corrections  for  chromatic  and  spherical  aberration. 
To  do  this  generally  requires  at  least  three  lenses,  and  photo- 
graphic objectives  thus  designed  {anastigmais)  may  give  a  sub- 
stantially flat  field  over  a  total  angle  of  50°  to  60°  with  corrections 
perfect  from  the  ordinary  photographic  standpoint. 

If  one  4emands  the  rigorous  precision  of  corrections  called  for 
in  astronomical  work,  the  possible  angle  is  very  much  reduced. 
Few  astrographic  lenses  cover  more  than  a  10°  or  15°  field,  and  the 
wider  the  relative  aperture  the  harder  it  is  to  get  an  anastigmat- 
ically  flat  field  free  of  material  errors.     Astigmatism  is  rarely 


THE  PROPERTIES  OF  OBJECTIVES  AND  MIRRORS        85 


noticeable  in  ordinary  telescopes,  but  is  sometimes  conspicuous  in 
eyepieces. 

Curvature  of  field  results  from  the  tendency  of  oblique  rays  in 
objectives,  otherwise  well  corrected,  to  come  to  shorter  focus 
than  axial  rays,  from  their  more  considerable  refraction  resulting 
from  greatly  increased  angles  of  incidence.  This  applies  to  bot 
the  astigmatic  image  surfaces,  which  are  concave  toward  the 
objective  in  all  ordinary  cases. 

Fortunately  both  these  faults  are  negligible  near  the  axis. 

They  are  both  proportional  to  — 7 —  where  u  is  the  obliquity  to 
the  axis  and  f  the  focal  length;  turn  up  with  serious  effect  in  wide 


li 


Fig.  57. — Steinheil  Triple  Objective.    Fig.  58. — ToUes  Quadruple  Objective. 


angled  lenses  such  as  are  used  in  photography,  but  may  generally 
be  forgotten  in  telescopes  of  the  ordinary  F  ratios,  like  F/12  to 
F/16.  So  also  one  may  commonly  forget  a  group  of  residual 
aberrations  of  higher  orders,  but  below  about  F/8  look  out  for 
trouble.  Objectives  of  wider  aperture  require  a  very  careful 
choice  of  special  glasses  or  the  sub-division  of  the  curvatures 
by  the  use  of  three  or  more  lenses  instead  of  two.  Fig.  57 
shows  a  cemented  triplet  of  Steinheil's  design,  with  a  crown  lens 
between  two  flints.  Such  triplets  are  made  up  to  about  4  inches 
diameter  and  of  apertures  ranging  from  jP/4  to  F/5. 

In  cases  of  demand  for  extreme  relative  aperture,  objectives 
composed  of  four  cemented  elements  have  now  and  then  been 
produced.  An  example  is  shown  in  Fig.  58,  a  four-part  objective 
of  1  inch  aperture  made  by  Tolles  years  ago  for  a  small  hand 
telescope.  Its  performance,  although  it  worked  at  F/4i,  was  re- 
ported to  be  excellent  even  up  to  75  diameters. 

The  main  difficulty  with  these  objectives  of  high  aperture  is 
the  relatively  great  curvature  of  field  due  to  short  focal  length 
which  prevents  full  utilization  of  the  improved  corrections  off 
the  axis. 


86 


THE  TELESCOPE 


Distortion  is  similarly  due  to  the  fact  that  magnification  is 
not  quite  the  same  for  rays  passing  at  different  distances 
from  the  axis.  It  varies  in  general  with  the  cube  of  the  distance 
from  the  axis,  and  is  usually  negligible  save  in  photographic 
telescopes,  ordinary  visual  fields  being  too  small  to  show  it 
conspicuously. 

Distortion  is  most  readily  avoided  by  adopting  the  form  of  a 
symmetrical  doublet  of  at  least  four  lenses  as  in  common  photo- 
graphic use.  No  simple  achromatic  pair  gives  a  field  wholly  free 
of  distortion  and  also  of  the  ordinary  aberrations,  except  very 
near  the  axis,  and  in  measuring  plates  taken  with  such  simple 
objectives  corrections  for  distortion  are  generally  required. 

At  times  it  becomes  necessary  to  depart  somewhat  from  the 
objective  form  which  theoretically  gives  the  least  aberrations  in 
order  to  meet  some  specific  requirement.  Luckily 
one  may  modify  the  ratios  of  the  curves  very  per- 
ceptibly without  serious  results.  The  aberrations 
produced  come  on  gradually  and  not  by  jumps. 

A  case  in  point  is  that  of  the  so-called  "bent" 
objective  in  which  the  curvatures  are  all  changed 
symmetrically,  as  if  one  had  put  his  fingers  on  the 
periphery  and  his  thumbs  on  the  centre  of  the  whole 
affair,  and  had  sprung  it  noticeably  one  way  or  the 
other. 

The  corrections  in  general  are  slightly  deteriorated 
but  the  field  may  be  in  effect  materially  flattened 
and  improved.  An  extreme  case  is  the  photographic 
Fig.  59-"Bent"  landscape  lens.  Figure  59  is  an  actual  example  from 
Objective,  g^  telescope  where  low  power  and  very  large  an- 
gular view  were  required.  The  objective  was  first  designed 
from  carefully  chosen  glass  to  meet  accurately  the  sine  condition. 
Even  so  the  field,  which  covered  an  apparent  angle  of  fully  40°, 
fell  off  seriously  at  the  edge. 

Bearing  in  mind  the  rest  of  the  system,  the  objective  was 
then  "bent"  into  the  form  given  by  the  dotted  lines,  and  the 
telescope  then  showed  beautiful  definition  clear  to  the  periphery 
of  the  field,  without  any  visible  loss  in  the  centre. 

This  spurious  flattening  cannot  be  pushed  far  without  getting 
into  trouble  for  it  does  not  cure  the  astigmatic  difference  of  focus, 
but  it  is  sometimes  very  useful.  Practically  curvature  of  field  is 
an  outstanding  error  that  cannot  be  remedied  in  objectives  re- 


THE  PROPERTIES  OF  OBJECTIVES  AND  MIRRORS        87 

quired  to  stand  high  magnifying  powers,  except  by  going  to  the 
anastigmatic  forms  similar  to  those  used  in  photography.^ 

Aside  from  curvature  the  chief  residual  error  in  objectives  is 
imperfection  of  achromatism.  This  arises  from  the  fact  that 
crown  and  flint  glasses  do  not  disperse  the  various  colors  quite  in 
the  same  ratio.  The  crown  gives  slightly  disproportionate 
importance  to  the  red  end  of  the  spectrum,  the  flint  to  the  violet 
end — the  so-called  "irrationality  of  dispersion." 

Hence  if  a  pair  of  lenses  match  up  accurately  for  two  chosen 
colors  like  those  represented  by  the  C  and  F  lines,  they  will  fail 
of  mutual  compensation  elsewhere.  Figure  60  shows  the  situa- 
tion. Here  the  spectra  from  crown  and  flint  glasses  are  brought 
to  exactly  the  same  extent  between  the  C  and  F  lines,  which 
serve  as  landmarks. 

Clearly  if  two  prisms  or  lenses  are  thus  adjusted  to  the  same 
refractions  for  C  and  F,  the  light  passing  through  the  combina- 
tion will  still  be  slightly  colored  in  virtue  of  the  differences  else- 
where in  the  spectrum.  These  residual  color  differences  produce 
what  is  known  as  the  "secondary  spectrum." 

What  this  does  in  the  case  of  an  achromatic  lens  may  be  clearly 
seen  from  the  figure;  C  and  F  having  exactly  the  same  refractions 
in  the  flint  and  crown,  come  to  the  same  focus.  For  D,  the  yellow 
line  of  sodium,  the  flint  lens  refracts  a  shade  the  less,  hence  is 
not  quite  powerful  enough  to  balance  the  crown,  which  therefore 
brings  D  to  a  focus  a  little  shorter  than  C  and  F.  On  the  other 
hand  for  A'  and  G',  the  flint  refracts  a  bit  more  than  the  crown, 
overbalances  it  and  brings  these  red  and  violet  rays  to  a  focus 
a  little  longer  than  the  joint  C  and  F  focus. 

1  The  curvature  of  the  image  is  the  thing  which  sets  a  limit  to  shortening 
the  relative  focus,  as  already  noted,  for  the  astigmatic  image  surfaces  as 
we  have  seen,  fall  rapidly  apart  away  from  the  axis,  and  both  curvatures 
are  considerable.  The  tangential  is  the  greater,  corresponding  roughly  to 
a  radius  notably  less  than  }i  the  focal  length,  while  the  radial  fits  a  radius 
of  less  than  %  this  length  with  all  ordinary  glasses,  given  forms  correcting 
the  ordinary  aberrations.  The  curves  are  concave  towards  the  objective 
except  in  "anastigmats"  and  some  objectives  having  bad  aberrations 
other^vise.  Their  approximate  curvatures  assuming  a  semiangular  aper- 
ture for  an  achromatic  objective  not  over  say  5°,  have  been  shown  to  be, 
to  focus  unity 

Pr  =  1  i ,  I ;  I  ,  and  pt  =  3  -\ ; ,) 

^^  —  J-  \n      n  /  p  —  i>  \n      n  / 

Pr  and  Pt  being  the  respective  reciprocals  of  the  radii.     The  surfaces  are  really 

somewhat  egg  shaped  rather  than  spherical  as  one  departs  from  the  axis. 


88 


THE  TELESCOPE 


The  difference  for  D  is  quite  small, 
roughly  about  3^^000  of  the  focal  length, 
while  the  red  runs  long  by  nearly  three 
times  that  amount,  the  violet  by  about 
four.  Towards  the  H  line  the  difference 
increases  rapidly  and  in  large  telescopes 
the  actual  range  of  focus  for  the  various 
colors  amounts  to  several  inches. 

This  difficulty  cannot  be  avoided  by 
any  choice  among  ordinary  pairs  of 
glasses,  which  are  nearly  alike  in  the 
matter  of  secondary  spectrum.  In  the 
latter  part  of  the  last  century  determined 
efforts  were  made  to  produce  glasses  that 
would  give  more  nearly  an  equal  run  of 
dispersion,  at  first  by  English  experimen- 
ters, and  then  with  final  success  by 
Schott  and  Abbe  at  Jena. 

Both  crown  and  flint  had  to  be  quite 
abnormal  in  composition,  especially  the 
latter,  and  the  pair  were  of  very  nearly 
the  same  refractive  index  and  with  small 
difference  in  the  quantity  v  which  we  have 
seen  determines  the  general  amount  of 
curvature.  Moreover  it  proved  to  be 
extremely  hard  to  get  the  crown  quite 
homogeneous  and  it  is  listed  by  Schott 
with  the  reservation  that  it  is  not  free 
from  bubbles  and  striae. 

Nevertheless  the  new  glasses  reduce 
the  secondary  spectrum  greatly,  to  about 
3^^  of  its  ordinary  value,  in  the  average. 
It  is  difficult  to  get  rid  of  the  spherical 
aberration,  however,  from  the  sharp 
curves  required  and  the  small  difference 
between  the  glasses,  and  it  seems  to  be 
impracticable  on  this  account  to  go  to 
greater  aperture  than  about  F/20. 

Figure  61  shows  the  deeply  curved 
form  necessary  even  at  half  the  relative 
aperture  usable  with  common  glasses.     At  F/20  the  secondary 


a 
o 
u 

THE  PROPERTIES  OF  OBJECTIVES  AND  MIRRORS        89 

spectrum  from  the  latter  is  not  conspicuous  and  Roe  (Pop.  Ast. 
18,  193),  testing  side  by  side  a  small  Steinheil  of  the  new  glasses, 
and  a  Clark  of  the  old,  of  almost  identical  size  and  focal  ratio, 
found  no  difference  in  their  practical  performance. 

Another  attack  on  the  same  problem  was  more  successfully 
made  by  H.  D.  Taylor.  Realizing  the  difficulty  found  with  a 
doublet  objective  of  even  the  best  matched  of  the  new  glasses,  he 
adopted  the  plan  of  getting  a  flint  of  exactly  the  right  dispersion 


Fig.  61.— Apochromatic  Doublet.        Fig.  62. — Apochromatic  Triplet. 


by  averaging  the  dispersions  of  a  properly  selected  pair  of  flints 
formed  into  lenses  of  the  appropriate  relative  curvatures. 

The  resulting  form  of  objective  is  made,  especially,  by  Cooke  of 
York,  and  also  by  Continental  makers,  and  carries  the  name  of 
"photo-visual"  since  the  exactness  of  corrections  is  carried  well 
into  the  violet,  so  that  one  can  see  and  photograph  at  the  same 
focus.  The  residual  chromatic  error  is  very  small,  not  above  ^■^ 
to  Ho  the  ordinary  secondary  spectrum. 

By  this  construction  it  is  practicable  to  increase  the  aperture 
to  F/12  or  F/10  while  still  retaining  moderate  curvatures  and 
reducing  the  residual  spherical  aberration.  There  are  a  round 
dozen  triplet  forms  possible,  of  which  the  best,  adopted  by  Taylor, 
is  shown  in  Fig.  62.  It  has  the  duplex  flint  ahead — first  a  baryta 
light  flint,  then  a  borosilicate  flint,  and  to  the  rear  a  special  light 
crown  The  two  latter  glasses  have  been  under  some  suspicion 
as  to  permanence,  but  the  difficulty  has  of  late  years  been  reported 
as  remedied.  Be  that  as  it  may,  neither  doublets  nor  triplets 
with   reduced   secondary    spectrum   have   come   into   any  large 


90  THE  TELESCOPE 

use  for  astronomical  purposes.  Their  increased  cost  is  con- 
siderable/ their  aperture  even  in  the  triplet,  rather  small  for 
astrophotography,  and  their  achromatism  is  still  lacking  the 
perfection  reached  by  a  mirror. 

The  matter  of  achromatism  is  further  complicated  by  the  fact 
that  objectives  are  usually  over-achromatized  to  compensate  for 
the  chromatic  errors  in  the  eye-piece,  and  especially  in  the  eye. 
As  a  general  rule  an  outstanding  error  in  any  part  of  an  optical 
system  can  be  more  or  less  perfectly  balanced  by  an  opposite  error 
anywhere  else  in  the  system — the  particular  point  chosen  being  a 
matter  of  convenience  with  respect  to  other  corrections. 

The  eye  being  quite  uncorrected  for  color  the  image  produced 
even  by  a  reflector  is  likely  to  show  a  colored  fringe  if  at  all  bright, 
the  more  conspicuous  as  the  relative  aperture  of  the  pupil 
increases.  For  low  power  eye-pieces  the  emerging  ray  may  quite 
fill  a  wide  pupil  and  then  the  chromatic  error  is  troublesome. 
Hence  it  has  been  the  custom  of  skilled  opticians,  from  the  time 
of  Fraunhofer,  who  probably  started  the  practice,  to  overdo  the 
correction  of  the  objective  just  a  little  to  balance  the  fault  of 
the  eye. 

What  actually  happens  is  shown  in  Fig.  63,  which  gives  the 
results  of  achromatization  as  practiced  by  some  of  the  world's 
adepts.  The  shortest  focus  is  in  the  yellow  green,  not  far  from 
the  line  D.  The  longest  is  in  the  violet,  and  F,  instead  of  coin- 
ciding in  focus  with  C  as  it  is  conventionally  supposed  to  do, 
actually  coincides  with  the  deep  and  faint  red  near  the  line  marked 
B.  Hence  the  visible  effect  is  to  lengthen  the  focus  for  blue 
enough  to  make  up  for  the  tendency  of  the  eye  in  the  other 
direction.  The  resulting  image  then  should  show  no  conspicuous 
rim  of  red  or  blue.  The  actual  adjustment  of  the  color  correc- 
tion is  almost  wholly  a  matter  of  skilled  judgment  but  Fig.  63 
shows  that  of  the  great  makers  to  be  quite  uniform.  The  smallest 
overcorrection  is  found  in  the  Grubb  objective,  the  largest  in  the 
Fraunhofer.  The  differences  seem  to  be  due  mainly  to  individual 
variations  of  opinion  as  to  what  diameter  of  pupil  should  be 
taken  as  typical  for  the  eye. 

The  common  practice  is  to  get  the  best  possible  adjustment  for 
a  fairly  high  power,  corresponding  to  a  beam  hardly  3^^4  inch  in 
diameter  entering  the  pupil. 

^  The  doublet  costs  about  one  and  a  half  times,  and  the  triplet  more  than 
twice  the  price  of  an  ordinary  achromatic  of  the  same  aperture. 


TEE  PROPERTIES  OF  OBJECTIVES  AND  MIRRORS       91 

In  any  case  the  bigger  the  pencil  of  rays  utilized  by  the  eye, 
i.e.,  the  lower  the  power,  the  more  overcorrection  must  be  pro- 
vided, so  that  telescopes  intended,  like  comet  seekers,  for  regular 


1. 

Fraunhofer 

2.  Clark 

3.  Steinheil 

4.  Hastings-Brashear 

5.  Grubb 

700 

B 

y 

>P^i 

/ 

^ 

<^ 

7^ 

'y 

//^ 

/ 

C 

650 

// 

<'' 

y^ 

/^ 

^  / 

^/ 

/ 

J/^/ 

f 

/// 

/'' 

/ 

600 

u 

to 

1 

1   \ 

I 

0) 

^550 

V 

\ 

iV 

\ 

E 

\ 

\' 

\ 

\ 

W 

^ 

\ 

N 

^N. 

^ 

N 

500 

F 

<: 

-^ 

^ 

*^ 

.^ 

^ 

v; 

-- 

— _^ 

450 

=^ 

.^ 

crtz 

■r:^ 

-— . 



~^ 

^ 

■"— 

-— 

=r- 

-"2" 

—■ 

rj^ 

-4 

^5 

g' 

'^ 

■~~-. 



3 

Jinn 

^ 

■^ 

■1 

60    40     20   -  0  +   20    40    60    80    100    20    40    60 

F 
100.000 

Fig.  63. — Achromatization  Curves  by  Various  Makers. 


use  with  low  powers  must  be  designed  accordingly,  either  as 
respects  objective  or  ocular. 

The  differences  concerned  in  this  chromatic  correction  for  power 
are  by  no  means  negligible  in  observing,  and  an  objective  actually 
conforming  to  the  C  to  F  correction  assumed  in  tables  of  optical 
glass  would  produce  a  decidedly  unpleasant  impression  when 


92  THE  TELESCOPE 

used  with  various  powers  on  bright  objects.  And  the  values  for 
V  impHed  in  the  actual  color  correction  are  not  immaterial  in 
computing  the  best  form  for  a  proposed  objective. 

1  is  from  Franunhofer's  own  hands,  the  instrument  of  9.6  inches 
aperture  and  170  inches  focus  in  the  Berlin  Observatory. 

2  The  Clark  refractor  of  the  Lowell  Observatory,  24  inches 
aperture  and  386  inches  focal  length.  This  is  of  the  usual  Clark 
form,  crown  ahead,  with  lenses  separated  by  about  3^^  of  their 
diameter. 

3  is  a  Steinheil  refractor  at  Potsdam  of  5.3  inches  aperture,  and 
85  inches  focus. 

4  is  from  the  fine  equatorial  at  Johns  Hopkins  University, 
designed  by  Professor  Hastings  and  executed  by  Brashear. 

The  objective  was  designed  with  special  reference  to  minimiz- 
ing the  spherical  aberration  not  only  for  one  chosen  wave  length 
but  for  all  others,  has  the  flint  lens  ahead,  aperture  9.4  inches, 
focal  length  142  inches,  and  the  lenses  separated  by  3^^  inch  in  the 
final  adjustment  of  the  corrections. 

5  is  from  the  Potsdam  equatorial  by  Grubb,  8.5  inches  aperture 
124  inches  focus 

The  great  similarity  of  the  color  curves  is  evident  at  a  glance, 
the  differences  due  to  variations  in  the  glass  being  on  the  whole 
much  less  significant  than  those  resulting  from  the  adjustment 
for  power. 

Really  very  little  can  be  done  to  the  color  correction  without 
going  to  the  new  special  glasses,  the  use  of  which  involves  other 
difficulties,  and  leaves  the  matter  of  adjustment  for  power  quite 
in  the  air,  to  be  brought  down  by  special  eye  pieces.  Now  and 
then  a  melting  of  glass  has  a  run  of  dispersion  somewhat  more 
favorable  than  usual,  but  there  is  small  chance  of  getting  large 
discs  of  special  characteristics,  and  the  maker  has  to  take  his 
chance,  minute  differences  in  chromatic  quality  being  far  less 
important  than  uniformity  and  good  annealing. 

Regarding  the  aberrations  of  mirrors  something  has  been  said 
in  Chap.  I,  but  it  may  be  well  here  to  show  the  practical  side 
of  the  matter  by  a  few  simple  illustrations. 

Figure  64  shows  the  simplest  form  of  concave  mirror — a 
spherical  surface,  in  this  instance  of  90°  aperture,  the  better  to 
show  its  properties.  If  light  proceeded  radially  outward  from  C, 
the  center  of  curvature  of  the  surface,  evidently  any  ray  would 
strike  the  surface  perpendicularly  as  at  a  and  would  be  turned 


THE  PROPERTIES  OF  OBJECTIVES  AND  MIRRORS        93 

squarely  back  upon  itself,  passing  again  through  the  center  of 
curvature  as  indicated  in  the  figure. 

A  ray,  however,  proceeding  parallel  to  the  axis  and  striking 
the  surface  as  at  bb  will  be  deflected  by  twice  the  angle  of  inci- 
dence as  is  the  case  with  all  reflected  rays.  But  this  angle  is 
measured  by  the  radius  Ch  from  the  center  of  curvature  and  the 
reflected  ray  makes  an  angle  ChF  with  the  radius,  equal  to  FCh. 


Fig.  64. — Reflection  from  Concave  Spherical  Mirror. 

For  points  very  near  the  axis  hF,  therefore,  equals  FC,  and  sub- 
stantially also  equals  cF.  Thus  rays  near  the  axis  and  parallel 
to  it  meet  at  F  the  focus  half,  way  from  c  to  C.  The  equivalent 
focal  length  of  a  spherical  concave  mirror  of  small  aperture  is 
therefore  half  its  radius  of  curvature. 

But  obviously  for  large  angles  of  incidence  these  convenient 
equalities  do  not  hold.  As  the  upper  half  of  the  figure  shows,  the 
ray  parallel  to  the  axis  and  incident  on  the  mirror  45°  away  at  e 
is  turned  straight  down,  for  it  falls  upon  a  surface  inclined  to  it  by 
45°and  is  therefore  deflected  by  90°,  cutting  the  axis  far  inside  the 
nominal  focus,  at  d.  Following  up  other  rays  nearer  the  axis  it 
appears  that  there  is  no  longer  a  focal  point  but  a  cusp-like  focal 
surface,  known  to  geometrical  optics  as  a  caustic  and  permitting 
no  well  defined  image. 

A  paraboloidal  reflecting  surface  as  in  Fig.  65  has  the  property 
of  bringing  to  a  single  point  focus  all  rays  parallel  to  its  axis  while 


94 


THE  TELESCOPE 


quite  failing  of  uniting  rays  proceeding  from  any  point  on  its 
axis,  since  its  curvature  is  changing  all  the  way  out  from  vertex  to 
periphery.  Here  the  parallel  rays  a,  a,  a,  a  meeting  the  surface 
are  reflected  to  the  focus  F,  while  in  a 
perfectly  symmetrical  way  the  prolongation 
of  these  rays  a',  a',  a',  a'  if  incident  on  the 
convex  surface  of  the  paraboloid  would  be 
reflected  in  R,  R',  R",  R'"  just  as  if  they 
proceeded  from  the  same  focus  F. 

The  difference  between  the  spherical  and 
parabolic  curves  is  well  shown  in  Fig.  66. 
Here  are  sections  of  the  former,  and  in 
dotted  lines  of  the  latter.  The  difference 
points  the  moral.  The  parabola  falls  away 
toward  the  periphery  and  hence  pushes 
outward  the  marginal  rays.  But  it  is  of  relatively  sharper 
curvature  near  the  center  and  pulls  in  the  central  to  meet  the 
marginal  portion.  In  the  actual  construction  of  parabolic 
mirrors  one  always  starts  with  a  sphere  which  is  easy  to  test 
for  precision  of  figure  at  its  center  of  curvature.  Then  the 
surface  may  be  modified  into  a  paraboloid  lessening  the  curva- 
ture towards  the  periphery,  or  by  increasing  the  curvature  toward 
the  center  starting  in  this  case  with  a  sphere  of  a  bit  longer  radius 
as  in  Fig.  66a. 


Fig.     65. — Reflection 
from  Paraboloid. 


Fig. 


66a.  Fig.  666. 

Variation  of  Paraboloid  from  Sphere. 


Practice  differs  in  this  respect,  either  process  leading  to  the 
same  result.  In  any  case  the  departure  from  the  spherical 
curve  is  very  slight — a  few  hundred  thousandths  or  at  most  ten 
thousandths  of  an  inch  depending  on  the  size  and  relative  focus 
of  the  mirror. 

Yet  this  small  variation  makes  all  the  difference  between 
admirable  and  hopelessly  bad  definition.  However  the  work  is 
done  it  is  guided  by  frequent  testing,  until  the  performance  shows 
that  a  truly  parabolic  figure  has  been  reached.  Its  attainment  is 
a  matter  of  skilled  judgment  and  experience. 

The  weak  point  of  the  parabolic  mirror  is  in  dealing  with  rays 


THE  PROPERTIES  OF  OBJECTIVES  AND  MIRRORS        95 

coming  in  parallel  but  oblique  to  the  axis.  Figure  67  shows  the 
situation  plainly  enough.  The  reflected  rays  a',  a"  no  longer  meet 
in  a  point  at  the  focus  F  but  inside  the  focus  for  parallel  rays,  at  / 
forming  a  surface  of  aberration.  The  practical  effect  is  that  the 
image  rapidly  deteriorates  as  the  star  passes  away  from  the  axis, 
taking  on  an  oval  character  that  suggests  a  bad  case  of  astigma- 
tism with  serious  complications  from  coma,  which  in  fact  is  sub- 
stantially the  case. 

Even  when  the  angular  aperture  is  very  small  the  focal  surface 
is  nevertheless  a  sphere  of  radius  equal  to  one  half  the  focal  length. 


Fig.  67. — Aberration  of  Parabolic  Mirror. 

and  the  aberrations  off  the  axis  increase  approximately  as  the 
square  of  the  relative  aperture,  and  directly  as  the  angular 
distance  from  the  axis. 

The  even  tolerably  sharp  field  of  the  mirror  is  therefore 
generally  small,  rarely  over  30'  of  arc  as  mirrors  are  customarily 
proportioned.  At  the  relative  aperture  usual  with  refractors,  say 
F/15,  the  sharp  fields  of  the  two  are  quite  comparable  in  extent. 

The   most  effective   help  for  the   usual  aberrations^   of  the 

1  A  very  useful  treatment  of  the  aberrations  of  parabolic  mirrors  by  Poor 
is  in  Ap.  J.  7,  114.  In  this  is  given  a  table  of  the  maximum  dimension  of  a 
star  disc  off  the  axis  in  reflectors  of  various  apertures.  This  table  condenses 
to  the  closely  approximate  formula 

_   lid 

a  —     p 

where  a  is  the  aberrational  diameter  of  the  star  disc,  in  seconds  of  arc,  d  the 

distance  from  the  axis   in  minutes  of  arc,  f  the  denominator  of  the  F  ratio 

(F/S  &c)  and  11,  a  constant.     Obviously  the  separating  power  of  a  tele- 

4.  "56 
scope  (see  Chap.  X)  being  substantially    "^     where  D  is  the  diameter  of 

objective  or  mirror  in  inches,  the  separating  power  will  be  impaired  when 

a>-^^ — .     In  the  photographic  case  the  critical  quantity  is  not -^y.—,  but 

the  maximum  image  diameter  tolerable  for  the  purpose  in  hand. 


96  THE  TELESCOPE 

mirror  is  the  adoption  of  the  Cassegrain  form,  by  all  odds  the 
most  convenient  for  large  instruments,  with  a  hyperboloidal 
secondary  mirror. 

The  hyperboloid  is  a  curve  of  very  interesting  optical  properties. 
Just  as  a  spherical  mirror  returns  again  rays  proceeding  from  its 
center  of  curvature  without  aberration,  and  the  paraboloid  sends 
from  its  focus  a  parallel  axial  beam  free  of  aberration,  or  returns 
such  a  beam  to  an  exact  focus  again,  so  a  hyperboloid.  Fig.  68, 
sends  out  a  divergent  beam  free  from  aberration  or  brings  it, 
returning,  to  an  exact  focus. 

Such  a  beam  a,  a,  a,  in  fact  behaves  as  if  it  came  from  and 
returned  to  a  virtual  conjugate  focus  F'  on  the  other  side  of  the 
hyperbolic  surface.  And  if  the  convex  side  be  reflecting,  con- 
verging rays  72,  R' ,  R" ,  falHng  upon  it  at  P,  P' ,  P" ,  as  if  headed 

for  the  virtual  focus  F,  will  actually 
be  reflected  to  F' ,  now  a  real  focus. 

This  surface  being  convex  its  ab- 
errations off  the  axis  are  of  opposite 
sign  to  those  due  to  a  concave  surface, 
and  can  in  part  at  least,  be  made  to 
compensate  the  aberrations  of  a  par- 
abolic main  mirror.  The  rationale  of 
the  operation  appears  from  comparison 
of  Figs.  67  and  68. 
Fig.  68.— Reflection  from  j^  the    former  the  oblique  rays  a, 

Hyperboloid.  •-    u    j     x  i,         i         j 

a  are  pitched  too  sharply  down. 
When  reflected  from  the  convex  surface  of  Fig.  68  as  a  converging 
beam  along  R,  R',  R",  they  can  nevertheless,  if  the  hyperbola  be 
properly  proportioned,  be  brought  down  to  focus  at  F'  conjugate 
to  F,  their  approximate   mutual   point  of  convergence. 

Evidently,  however,  this  compensation  cannot  be  complete  over 
a  wide  angle,  when  F'  spreads  into  a  surface,  and  the  net  result  is 
that  while  the  total  aberrations  are  materially  reduced  there  is 
some  residual  coma  together  with  some  increase  of  curvature  of 
field,  and  distortion.  Here  just  as  in  the  parabolizing  of  the  large 
speculum  the  construction  is  substantially  empirical,  guided  in 
the  case  of  a  skilled  operator  by  a  sort  of  insight  derived  from 
experience. 

Starting  from  a  substantially  spherical  convexity  of  very 
nearly  the  required  curvature  the  figure  is  gradually  modified 
as  in  the  earlier  example  until  test  with  the  truly  parabolic  mirror 


THE  PROPERTIES  OF  OBJECTIVES  AND  MIRRORS        97 

shows  a  flawless  image  for  the  combination.  The  truth  is  that 
no  conic  surface  of  revolution  save  the  sphere  can  be  ground  to 
true  figure  by  any  rigorous  geometrical  method.  The  result 
must  depend  on  the  skill  with  which  one  by  machine  or  hand 
can  gauge  minute  departures  from  the  sphere. 

Attempts  have  been  made  by  the  late  Professor  Schwarzchild 
and  others  to  improve  the  corrections  of  reflectors  so  as  to  increase 
the  fleld  but  they  demand  either  very  difficult  curvatures  imposed 
on  both  mirrors,  or  the  interposition  of  lenses,  and  have  thus  far 
reached  no  practical  result. 

References 
Schwarzchild:  Untersuchungen  "S!,  Geom.,  Opt.  II. 
Sampson  Observatory  36,  248. 

Coddington:  "Reflexion  and  Refraction  of  Light." 
Herschel:  "Light." 
Taylor:  "AppUed  Optics." 
Southall:  "Geometrical  Optics." 

Martin:  Ann.  Sci.  de  I'Ecole  Normale,  1877,  Supplement. 
Moser:  Zeit.  f.  Instrumentenkunde,  1887. 
Harting:  Zeit.  f.  Inst.,  1899. 
Harting:  Zeit.f.  Inst,  1898. 
von  Hoegh:  Zeit.  f.  Inst.,  1899. 
Steinheil  &  Voit:  "Applied  Optics." 

Collected  Researches,  National  Physical  Laboratory,  Vol.  14. 
Gleichen:  "Lehrbuch  d.  Geometrische  Optik." 

Note. — In  dealing  with  optical  formulae  look  out  for  the  alge- 
braic signs.  Writers  vary  in  their  conventions  regarding  them  and 
it  sometimes  is  as  difficult  to  learn  how  to  apply  a  formula  as 
to  derive  it  from  the  start.  Also,  especially  in  optical  patents, 
look  out  for  camouflage,  as  omitting  to  specify  an  optical  constant, 
giving  examples  involving  glasses  not  produced  by  any  manufac- 
turer, and  even  specifying  curves  leading  to  absurd  properties. 
It  is  a  good  idea  to  check  up  the  achromatization  and  focal  length 
before  getting  too  trustful  of  a  numerical  design. 


CHAPTER  V 
MOUNTINGS 

A  steady  and  convenient  mounting  is  just  as  necessary  to  the 
successful  use  of  the  telescope  as  is  a  good  objective.  No  satis- 
factory observations  for  any  purpose  can  be  made  with  a  tele- 
scope unsteadily  mounted  and  not  provided  with  adjustments 
enabling  it  to  be  moved  smoothly  and  easily  in  following  a 
celestial  object. 

Broadly,  telescope  mounts  may  be  divided  into  two  general 
classes,  alt-azimuth  and  equatorial.  The  former  class  is,  as  its 
name  suggests,  arranged  to  be  turned  in  azimuth  about  a  vertical 
axis,  and  in  altitude  about  a  horizontal  axis.  Such  a  mounting 
may  be  made  of  extreme  simplicity,  but  obviously  it  requires  two 
motions  in  order  to  follow  up  any  object  in  the  field,  for  the 
apparent  motion  of  the  heavenly  bodies  is  in  circles  about  the 
celestial  pole  as  an  axis,  and  consequently  inclined  from  the 
vertical  by  the  latitude  of  the  place  of  observation. 

Pointing  a  telescope  with  motions  about  a  vertical  and  hori- 
zontal axis  only,  therefore  means  that,  as  a  star  moves  in  its 
apparent  path,  it  will  drift  away  from  the  telescope  both  in 
azimuth  and  in  altitude,  and  require  to  be  followed  by  a  double 
motion. 

Alt-azimuth  mounts  may  be  divided  into  three  general  groups 
according  to  their  construction.  The  first  and  simplest  of  them 
is  the  pillar-and-claw  stand  shown  in  Figure  69.  This  consists 
of  a  vertical  pillar  supported  on  a  strong  tripod,  usually  of  brass 
or  iron,  and  provided  at  its  top  with  a  long  conical  bearing 
carrying  at  its  upper  extremity  a  hinged  joint,  bearing  a  bar  to 
which  the  telescope  is  screwed  as  shown  in  the  illustration. 

If  properly  made  the  upper  joint  comprises  a  circular  plate 
carrying  the  bar  and  held  between  two  cheek  pieces  with  means 
for  taking  up  wear,  and  providing  just  enough  friction  to  permit 
of  easy  adjustment  of  the  telescope,  which  can  be  swung  in 
altitude  from  near  the  zenith  to  the  horizon  or  below,  and  turned 
around  its  vertical  axis  in  any  direction. 

98 


MOUNTINGS 


99 


When  well  made  a  stand  of  this  kind  is  steady  and  smooth 
working,  readily  capable  of  carrying  a  telescope  up  to  about  3 
inches  aperture.  It  needs  for  its  proper  use  a  firm  sub-support 
for  the  three  strong  hinged  legs  of  the  pillar.     This  is  conveniently 


Fig.  69. — Table  Mount  with  Slow  Motion. 


made  as  a  very  solid  stool  with  spreading  legs,  or  a  plank  of 
sufl&cient  size  may  be  firmly  bolted  to  a  well  set  post. 

Thus  arranged  the  mount  is  a  very  serviceable  one  for  small 
instruments.  Its  stability,  however,  depends  on  the  base  upon 
which  it  is  set.  The  writer  once  unwisely  attempted  to  gain 
convenience  by  removing  the  legs  of  the  stand  and  screwing  its 
body  firmly  upon  a  very  substantial  tripod.  The  result  was  a 
complete  failure  in  steadiness,  owing  to  the  rather  long  lever  arm 
furnished  by  the  height  of  the  pillar;  and  the  instrument,  which 
had  been  admirably  steady  originally,  vibrated  abominably 
when  touched  for  focussing. 


100 


THE  TELESCOPE 


The  particular  stand  here  shown  is  furnished  with  a  rack 
motion  in  altitude  which  is  a  considerable  convenience  in  follow- 
ing. More  rarely  adjustable  steadying  rods  attached  to  the 
objective  end  of  the  instrument  are  brought  down  to  its  base, 
but  for  a  telescope  large  enough  to  require  this  a  better  mount  is 
generally  desirable. 

Now  and  then  an  alt-azimuth  head  of  just  the  sort  used  in  the 
pillar-and-claw  stand  is  actually  fitted  on  a  tall  tripod,  but  such 


Fig.  70.— Altazimuth  Mount,  Clark  Type  T. 


an  arrangement  is  usually  found  only  in  cheap  instruments  and 
for  such  tripod  mountings  other  fittings  are  desirable. 

The  second  form  of  alt-azimuth  mount,  is  altogether  of  more 
substantial  construction.  The  vertical  axis,  usually  tapered  and 
carefully  ground  in  its  bearings,  carries  an  oblique  fork  in 
which  the  telescope  tube  is  carried  on  trunnions  for  its  vertical 
motion.  The  inclination  of  the  forked  head  enables  the  tele- 
scope to  be  pointed  directly  toward  the  zenith  and  the  whole 
mount  forms  the  head  of  a  well  made  tripod. 

Figure  70  shows  an  excellent  type  of  this  form  of  mount  as 


MOUNTINGS 


101 


used  for  the  Clark  Type  T  telescope,  designed  for  both  terrestrial 
and  astronomical  observation.  In  this  particular  arrangement 
the  telescope  lies  in  an  aluminum  cradle  carried  on  the  trunnions, 


Fig.  71. — Altazimuth  with  Full  Slow  Motions. 


from  which  it  can  be  readily  removed  by  loosening  the  thumb 
screws  and  opening  the  cradle. 

It  can  also  be  set  longitudinally  for  balance  in  the  cradle  if  any 
attachments  are  to  be  placed  upon  either  end.  Here  the  adjust- 
ment for  the  height  of  the  instrument  is  provided  both  in  the 


102  THE  TELESCOPE 

spread  of  the  tripod  and  in  the  adjustable  legs.  So  mounted  a 
telescope  of  3  or  4  inches  aperture  is  easily  handled  and  capable 
of  rendering  very  good  service  either  for  terrestrial  or  celestial 
work. 

Indeed  the  Clarks  have  made  instruments  up  to  6  inches 
aperture,  mounted  for  special  service  in  this  simple  manner. 
For  celestial  use  where  fairly  high  powers  may  be  required  this 
or  any  similar  mount  can  be  readily  furnished  with  slow  motions 
either  in  azimuth  or  altitude  or  both. 

Figure  71  shows  a  43-^  inch  telescope  and  mount  by  Zeiss  thus 
equipped.  Some  alt-azimuth  mounts  are  also  provided  with  a 
vertical  rack  motion  to  bring  the  telescope  to  a  convenient  height 
without  disturbing  the  tripod.  A  good  alt-azimuth  mount  such 
as  is  shown  in  Figs.  70  and  71  is  by  no  means  to  be  despised 
for  use  with  telescopes  of  3  or  4  inch  aperture. 

The  sole  inconvenience  to  be  considered  is  that  of  the  two 
motions  required  in  following.  With  well  fitted  slow  motions 
this  is  not  really  serious  for  ordinary  observing  with  moderate 
powers,  for  one  can  work  very  comfortably  up  to  powers  of 
150  or  200  diameters  keeping  the  object  easily  in  view;  but  with 
the  higher  powers  in  which  the  field  is  very  small,  only  a  few 
minutes  of  arc,  the  double  motion  becomes  rather  a  nuisance  and 
it  is  extremely  inconvenient  even  with  low  powers  in  sweeping 
for  an  object  the  place  of  which  is  not  exactly  known. 

There  are  in  fact  two  distinct  kinds  of  following  necessary 
in  astronomical  observations.  First,  the  mere  keeping  of  the 
object  somewhere  in  the  field,  and  second,  holding  it  somewhat 
rigorously  in  position,  as  in  making  close  observations  of  detail 
or  micrometrical  measurements.  When  this  finer  following 
is  necessary  the  sooner  one  gets  away  from  alt-azimuth  mounts 
the  better. 

Still  another  form  of  alt-azimuth  mount  is  shown  in  Fig.  72 
applied  for  a  Newtonian  reflector  of  6  or  8  inches  aperture. 
Here  the  overhung  fork  carrying  the  tube  on  trunnions  is  sup- 
ported on  a  stout  fixed  tripod,  to  which  it  is  pivoted  at  the  front, 
and  it  is  provided  at  the  rear  with  a  firm  bearing  on  a  sector  borne 
by  the  tripod. 

At  the  front  a  rod  with  sliding  coarse,  and  screw  fine,  adjust- 
ment provides  the  necessary  motion  in  altitude.  The  whole  fork 
is  swung  about  its  pivot  over  the  sector  bearing  by  a  cross  screw 
turned  by  a  rod  with  a  universal  joint. 


MOUNTINGS 


103 


This  mount  strongly  suggests  the  original  one  of  Hadley,  Fig. 
16,  and  is  most  firm  and  serviceable.  A  reflector  thus  mounted  is 
remarkably  convenient  in  that  the  eyepiece  is  always  in  a  most 
accessible  position,  the  view  always  horizontal,  and  the  adjust- 
ments always  within  easy  reach  of  the  observer. 


Fig.  72. — Altazimuth  Newtonian  Reflector. 


Whenever  it  is  necessary  to  follow  an  object  closely,  as  in 
using  a  micrometer  and  some  other  auxiliaries,  the  alt-azimuth 
mount  is  troublesome  and  some  modification  adjustable  by  a 
single  motion,  preferably  made  automatic  by  clockwork,  becomes 
necessary. 

The  first  step  in  this  direction  is  a  very  simple  one  indeed. 


104 


THE  TELESCOPE 


Suppose  one  were  to  tilt  the  azimuth  axis  so  that  it  pointed  to  the 
celestial  pole,  about  which  all  the  stars  appear  to  revolve.  Then 
evidently  the  telescope  being  once  pointed,  a  star  could  be  fol- 
lowed merely  by  turning  the  tube  about  this  tilted  axis.  Of 
course  one  could  not  easily  reach  some  objects  near  the  pole 
without,  perhaps,  fouling  the  mount,  but  in  general  the  sky  is 

within  reach  and  a  single 
motion  follows  the  star,  very 
easily  if  the  original  mount 
had  a  slow  motion  in  azimuth. 
This  is  in  fact  the  simplest 
form  of  equatorial  mount, 
sometimes  called  parallactic. 
Figure  73  shows  the  principle 
applied  to  a  small  reflector. 
An  oblique  block  with  its  angle 
adjusted  to  the  co-latitude  of 
the  place  drops  the  vertical 
axis  into  line  with  the  pole, 
and  the  major  part  of  the  ce- 
lestial vault  is  then  within 
easy  reach. 

It  may  be  regarded  as  the 
transition  step  from  the  alt- 
azimuth to  the  true  equato- 
rial. It  is  rarely  used  for 
refractors,  and  the  first  at- 
tempt at  a  real  equatorial 
mount  was  in  fact  made  by 
James     Short    F.    R.    S.    in 

Fig.  73. — Parallactic  Mount  for  Reflector,  mounting     SOme    of   his   Small 

Gregorians.^  As  a  matter  of 
record  this  is  shown,  from  Short's  own  paper  before  the  Royal 
Society  in  1749,  in  Fig.  74. 

A  glance  shows  a  stand  apparently  most  complicated,  but 
closer  examination  discloses  that  it  is  merely  an  equatorial 
on  a  table  stand  with  a  sweep  in  declination  over  a  very  wide  arc, 

^Instruments  with  a  polar  axis  were  used  by  Scheiner  as  early  as  1627; 
by  Roemer  about  three  quarters  of  a  century  later,  and  previously  had 
been  employed,  using  sights  rather  than  telescopes,  by  the  Chinese;  but 
these  were  far  from  being  equatorials  in  the  modern  sense. 


MOUNTINGS 


105 


and  quite  complete  arrangements  for  setting  to  the  exact  latitude 
and  azimuth.  The  particular  instrument  shown  was  of  4  inches 
aperture  and  about  18  inches  long  and  was  one  of  several 
produced  by  Short  at  about  this  epoch. 


Fig.  74. —  Short's  Equatorial  Mount. 


In  the  instrument  as  shown  there  is  first  an  azimuth  circle 
A  A  supported  on  a  base  B  B  B  B  having  levelling  screws  in  the 
feet.  Immediately  under  the  azimuth  circle  is  mounted  a  com- 
pass needle  for  approximate  orientation,  and  the  circle  is  adjust- 
able by  a  tangent  screw  C. 


106  THE  TELE  SCOPE 

Carried  by  the  azimuth  circle  on  a  bearing  supported  by  four 
pillars  is  a  latitude  circle  D  D  for  the  adjustment  of  the  polar 
axis,  with  a  slow  motion  screw  E.  The  latitude  circle  carries  a 
right  ascension  circle  F  F,  with  a  slow  motion  G,  and  this  in  turn 
carries  on  four  pillars  the  declination  circle  H  H,  and  its  axis 
adjustable  by  the  slow  motion  K. 

To  this  declination  circle  is  secured  the  Gregorian  reflector 
L  L  which  serves  as  the  observing  telescope.  All  the  circles  are 
provided  with  verniers  as  well  as  slow  motions.  And  the 
instrument  is,  so  to  speak,  a  universal  one  for  all  the  purposes  of 
an  equatorial,  and  when  the  polar  axis  is  set  vertical  equally 
adapted  for  use  as  a  transit  instrument,  theodolite,  azimuth 
instrument,  or  level,  since  the  circles  are  provided  with  suitable 
levels. 

This  mount  was  really  a  very  neat  and  complete  piece  of  work 
for  the  purpose  intended,  although  scarcely  suitable  for  mounting 
any  but  a  small  instrument.  A  very  similar  construction  was 
used  later  by  Ramsden  for  a  small  refractor. 

It  is  obvious  that  the  reach  of  the  telescope  when  used  as  an 
equatorial  is  somewhat  limited  in  the  mount  just  described. 
In  modern  constructions  the  telescope  is  so  mounted  that  it  may 
be  turned  readily  to  any  part  of  the  sky,  although  often  the 
polar  axis  must  be  swung  through  180°  in  order  to  pass  freely 
from  the  extreme  southern  to  the  extreme  northern  heavens. 

The  two  motions  necessary  are  those  in  right  ascension  to 
follow  the  heavenly  bodies  in  their  apparent  course,  and  in 
declination  to  reach  an  object  at  any  particular  angular  distance 
from  the  pole. 

There  are  always  provided  adjustments  in  azimuth  and  for 
latitude  over  at  least  a  small  arc,  but  these  adjustments  are 
altogether  rudimentary  as  compared  with  the  wide  sweep  given 
by  Short. 

The  fundamental  construction  of  the  equatorial  involves 
two  axes  working  at  right  angles  positioned  like  a  capital  T. 

The  upright  of  the  T  is  the  polar  axis,  fitted  to  a  sleeve  and 
bearing  the  cross  of  the  T,  which  is  hollow  and  provides  the 
bearing  for  the  declination  aixis,  which  again  carries  at  right 
angles  to  itself  the  tube  of  the  telescope. 

When  the  sleeve  which  carries  the  upright  of  the  T  points  to 
the  pole  the  telescope  tube  can  evidently  be  swung  to  cover  an 
object  at  any  altitude,  and  can  then  be  turned  on  its  polar  axis 


MOUNTINGS 


107 


so  as  to  follow  that  object  in  its  apparent  diurnal  motion.  The 
front  fork  of  a  bicycle  set  at  the  proper  angle  with  a  cross  axis 
replacing  the  handle  bars  has  more  than  once  done  good  service 


Fig.  75. — Section  of  Modern  Equatorial. 

in  an  emergency.     Figure  75  shows  in  section  a  modern  equatorial 
mount  for  a  medium  sized  telescope. 

The  mounting  shown  in  Fig.  75,  by  Zeiss,  is  thoroughly  typical 
of  recent  practice  in  instruments  of  moderate  size.  The  general 
form  of  this  equatorial  comes  straight  down  to  us  from  Fraun- 


108  THE  TELESCOPE 

hofer's  mounting  of  the  Dorpat  instrument.  It  consists  essen- 
tially of  two  axes  crossed  exactly  at  right  angles. 

P,  the  polar  axis,  is  aligned  exactly  with  the  pole,  and  is  sup- 
ported on  a  hollow  iron  pier  provided  at  its  top  with  the  latitude 
block  L  to  which  the  bearings  of  P  are  bolted.  D  the  declination 
axis  supports  the  telescope  tube  T. 

The  tube  is  counter-poised  as  regards  the  polar  axis  by  the 
weight  a,  and  as  regards  the  declination  axis  by  the  weights  b  b. 
At  A,  the  upper  section  of  the  pier  can  be  set  in  exact  azimuth 
by  adjusting  screws,  and  at  the  base  of  the  lower  section  the 
screws  at  B.  B.  allow  some  adjustment  in  latitude.  To  such 
mere  rudiments  are  the  azimuth  and  altitude  circles  of  Short's 
mount  reduced. 

At  the  upper  end  of  the  polar  axis  is  fitted  the  gear  wheel  g, 
driven  by  a  worm  from  the  clock-work  at  C  to  follow  the  stars 
in  their  course.  At  the  lower  end  of  the  same  axis  is  the  hour 
circle  h,  graduated  for  right  ascension,  and  a  hand  wheel  for 
quick  adjustment  in  R.  A. 

At  d  is  the  declination  circle,  which  is  read,  and  set,  by  the  tele- 
scope t  with  a  right  angled  prism  at  its  upper  end,  which  saves 
the  observer  from  leaving  the  eye  piece  for  small  changes. 

F  is  the  usual  finder,  which  should  be  applied  to  every  telescope 
of  3  inches  aperture  and  above.  It  should  be  of  low  power,  with 
the  largest  practicable  field,  and  has  commonly  an  aperture  3^^  or 
3-^  that  of  the  main  objective,  big  enough  to  pick  up  readily 
objects  to  be  examined  and  by  its  coarse  cross  wires  to  bring 
them  neatly  into  the  field.  At  m  and  n  are  the  clamping  screws 
for  the  right  ascension  and  declination  axes  respectively,  while 
o  and  p  control  the  respective  tangent  screws  for  fine  adjustment 
in  R.  A.  and  Dec.  after  the  axes  are  clamped.  This  mount  has 
really  all  the  mechanical  refinements  needed  in  much  larger 
instruments  and  represents  the  class  of  permanently  mounted 
telescopes  used  in  a  fixed  observatory. 

The  ordinary  small  telescope  is  provided  with  a  mount  of 
the  same  general  type  but  much  simpler  and,  since  it  is  not  in  a 
fixed  observatory,  has  more  liberal  adjustments  in  azimuth  and 
altitude  to  provide  for  changes  of  location.  Figure  76  shows  in 
some  detail  the  admirable  portable  equatorial  mounting  used 
by  the  Clarks  for  instruments  up  to  about  5  or  6  inches  aperture. 

Five  inches  is  practically  the  dividing  line  between  portable 
and  fixed  telescopes.     In  fact  a  5  inch  telescope  of  standard  con- 


MOUNTINGS 


109 


struction  with  equatorial  mounting  is  actually  too  heavy  for 
practical  portability  on  a  tripod  stand.  The  Clarks  have  turned 
out  really  portable  instruments  of  this  aperture,  of  relatively 
short  focus  and  with  aluminum  tube  fitted  to  the  mounting 
standard  for  a  4  inch  telescope,  but  the  ordinary  5  inch  equipment 
of  the  usual  focal  length  deserves  a  permanent  placement. 
In  this  mount  the  short  tapered  polar  axis  P  is  socketed  between 


Fig.  76.— Clark  Adjustable  Equatorial  Mount. 


the  cheeks  A,  and  tightened  in  any  required  position  by  the  hand 
screws  B.  B.  The  stout  declination  axis  D  bears  the  telescope 
and  the  counterweight  C.  Setting  circles  in  R.  A.  and  Dec,  p 
and  d  respectively,  are  carried  on  the  two  axes,  and  each  axis 
has  a  worm  wheel  and  tangent  screw  operated  by  a  universal 
joint  to  give  the  necessary  slow  motion. 

The  worm  wheels  carry  their  respective  axes  through  friction 
bearings  and  the  counter  poising  is  so  exact  that  the  instrument 
can  be  quickly  swung  to  any  part  of  the  sky  and  the  slow  motion 
picked  up  on  the  instant.  The  wide  sweep  of  the  polar  axis 
allows  immediate  conversion  into  an  alt-azimuth  for  terrestrial 


110  THE  TELESCOPE 

use,  or  adjustment  for  any  latitude.  A  graduated  latitude  arc  is 
customarily  engraved  on  one  of  the  check  pieces  to  facilitate 
this  adjustment. 

Ordinarily  portable  equatorials  on  tripod  mounts  are  not 
provided  with  circles,  and  have  only  a  single  slow  motion,  that 
in  R.  A.  A  declination  circle,  however,  facilitates  setting  up 
the  instrument  accurately  and  is  convenient  for  locating  an 
object  to  be  swept  for  in  R.  A.  which  must  often  be  done  if  one 
has  not  sidereal  time  at  hand.  In  Fig.  76  a  thumb  screw  under- 
neath the  tripod  head  unclamps  the  mount  so  that  it  may  be  at 
once  adjusted  in  azimuth  without  shifting  the  tripod. 

As  a  rule  American  stands  for  fixed  equatorials  have  the  clock 
drive  enclosed  in  the  hollow  pillar  which  carries  the  equatorial 
head  as  shown  in  the  reflector  of  Fig.  35,  and  in  the  Clark 
mount  for  refractors  of  medium  size  shown  in  Fig.  77.  Here  a 
neat  quadrangular  pillar  carries  an  equatorial  mounting  in 
principle  very  much  like  Fig.  76,  but  big  enough  to  carry  tele- 
scopes of  8  to  10  inches  aperture.  It  has  universal  adjustment  in 
latitude,  so  that  it  can  be  used  in  either  hemisphere,  the  clock 
and  its  driving  weight  are  enclosed  in  the  pillar  and  slow  motions 
are  provided  for  finding  in  R.  A.  and  Dec.  The  adjustment  in 
azimuth  is  made  by  moving  the  pillar  on  its  base-plate,  which  is 
bolted  to  the  pier.  The  convenient  connections  for  accurate 
following  and  the  powerful  clock  make  the  mount  especially 
good  for  photographic  telescopes  of  moderate  size  and  the  whole 
equipment  is  most  convenient  and  workmanlike.  It  is  worth 
noting  that  the  circles  are  provided  with  graduations  that  are 
plain  rather  than  minute,  in  accordance  with  modern  practice. 
In  these  days  of  celestial  photography  equatorials  are  seldom 
used  for  determining  positions  except  with  the  micrometer,  and 
graduated  circles  therefore,  primarily  used  merely  for  finding, 
should  be,  above  all  things,  easy  to  read. 

All  portable  mounts  are  merely  simplifications  of  the  observa- 
tory type  of  Fig.  75,  which,  with  the  addition  of  various  labor 
saving  devices  is  applied  to  nearly  all  large  refractors  and  to  many 
reflectors  as  well. 

There  is  a  modified  equatorial  mount  sometimes  known  as 
the  ''English"  equatorial  in  which  the  polar  axis  is  long  and 
supported  on  two  piers  differing  enough  in  height  to  give  the 
proper  latitude  angle,  the  declination  axis  being  about  midway 
of  the  polar  axis.     A  bit  of  the  sky  is  cut  off  by  the  taller  pier, 


MOUNTINGS 


111 


and  the  type  is  not  especially  advantageous  unless  in  supporting 
a  very  heavy  instrument,  too  heavy  to  be  readily  overhung  in 
the  usual  way. 


Fig.  77. — Universal  Observatory  Mount  (Clark  9-inch). 

In  such  case  some  form  of  the  "English"  mounting  is  very 
important   to   securing  freedom  from  flexure  and  thereby  the 


112 


THE  TELESCOPE 


perfection  of  driving  in  R.  A.  so  important  to  photographic  work. 
The  72  inch  Dominion  Observatory  reflector  and  the  100  inch 
Hooker  telescope  at  Mt.  Wilson  are  thus  mounted,  the  former 


Fig.  78. — English  Equatorial  Mount  (Hooker  100-inch  Telescope). 


on  a  counterpoised  declination  axis  crosswise  the  polar  axis,  the 
original  "Enghsh"  type;  the  latter  on  trunnions  within  a  long 
closed  fork  which  carries  the  polar  bearings  at  its  ends. 


MOUNTINGS 


113 


Figure  78  shows  the  latter  instrument,  of  100  inches  clear  aper- 
ture and  of  42  feet  principal  focal  length,  increased  to  135  feet 
when  used  as  a  Cassegrainian.  It  is  the  immense  stability  of  this 
mount  that  has  enabled  it  to  carry  the  long  cross  girder  bearing 
the  interferometer  recently  used  in  measuring  the  diameters  of 


Fig.  79. — English  Equatorial  Mount  (72-inch  Dominion  Observatory  Telescope) . 

the  stars.  Note  the  mercury-flotation  drum  at  each  end  of  the 
polar  axis.  The  mirrors  were  figured  by  the  skillful  hands  of 
Mr.  Ritchey. 

Figure  79  gives  in  outline  the  proportions  and  mounting  of 
the  beautiful  instrument  in  service  at  the  Dominion  Observatory, 
near  Victoria,  B.  C.  The  mirror  and  its  auxiliaries  were  figured 
by  Brashear  and  the  very  elegant  mounting  was  by  Warner  and 
Swasey.  The  main  mirror  is  of  30  feet  principal  focus.  The  20 
inch  hyperboloidal  mirror  extends  the  focus  as  a  Cassagrainian  to 
108  feet.  The  mechanical  stability  of  these  English  mounts 
for  very  large  instruments  has  been  amply  demonstrated  by 


114 


THE  TELESCOPE 


this,  as  by  the  Hooker  100  inch  reflector.  They  suffer  less  from 
flexure  than  the  Fraunhofer  mount  where  great  weights  are  to  be 
carried,  although  the  latter  is  more  convenient  and  generally 
useful  for  instruments  of  moderate  size.  It  is  hard  to  say  too 
much  of  the  mechanical  skill  that  has  made  these  two  colossal 


Fig.  80. — Astrographic  Mount  with  Bent  Pier. 

telescopes  so  completely  successful  as  instruments  of  research. 
The  inconvenience  of  having  to  swing  the  telescope  tube  to 
clear  the  pier  at  certain  points  in  the  R.  A.  following  is  often  a 
serious  nuisance  in  photographic  work  requiring  long  exposures, 
and  may  waste  valuable  time  in  visual  work.     Several  recent 


MOUNTINGS 


115 


forms  of  equatorial  mount  have  therefore  been  devised  to  allow 
the  telescope  complete  freedom  of  revolution  in  R.  A.,  swinging 
clear  of  everything. 

One  such  form  is  shown  in  Fig.  80  which  is  a  standard  astro- 
graphic  mount  for  a  Brashear  doublet  and  guiding  telescope. 
The  pier  is  strongl}^  overhung  in  the  direction  of  the  polar  axis 
far  enough  to  allow  the  instrument  to  follow  through  for  any 


Fig.  81. — Open  Fork  Mounting. 

required  period,  even  to  resuming  operations  on  another  night 
without  a  shift  of  working  position. 

Another  form,  even  simpler  and  found  to  be  extremely  satis- 
factory even  for  rather  large  instruments,  is  the  open  polar  fork 
mount.  Here  the  polar  axis  of  an  ordinary  form  is  continued  by 
a  wide  and  stiff  casting  in  the  form  of  a  fork  within  which  the 
tube  is  carried  on  substantial  trunnions,  giving  it  complete 
freedom  of  motion. 

The  open  fork  mount  in  its  simplest  form,  carrying  a  heliostat 
mirror,  is  shown  in  Fig.  81.  Here  A  is  the  fork,  B  the  polar 
axis,  carried  on  an  adjustable  sector  for  variation  in  latitude,  C 
the  declination  axis  carrying  the  mirror  D  in  its  cell,  E  the  slow 


116 


THE  TELESCOPE 


motion  in  declination,  and  F  that  in  R.  A.  Both  axes  can  be 
undamped  for  quick  motion  and  the  R.  A.  axis  can  readily  be 
driven  by  clock  or  electric  motor. 

The  resemblance  to  the  fully  developed  English  equatorial 
mount  of  Fig.  78  is  obvious,  but  the  present  arrangement  gives 
entirely  free  swing  to  a  short  instrument,  is  conveniently  adjust- 
able, and  altogether  workmanlike.  It  can  easily  carry  a  short 
focus  celestial  camera  up  to  6  or  8  inches  aperture  or  a  reflector  of 
4  or  5  feet  focal  length. 

In  Fig.  173,  Chap.  X  a  pair  of  these  same  mounts  are  shown 
at  Harvard  Observatory.     The  nearer  one,  carrying  a  celestial 


Fig.  82. — Mounting  of  Mt.  Wilson  60- 
inch  Reflector. 


Fig.  83. — The  60-inch  as  Casse- 
grainian,  F  =  100'. 


camera,  is  exposed  to  view.  It  is  provided  with  a  slow  motion 
and  clamp  in  declination,  and  with  an  electric  drive  in  R.  A., 
quickly  undamped  for  swinging  the  camera.  It  works  very 
smoothly,  its  weight  is  taken  by  a  very  simple  self  adjusting 
thrust  bearing  at  the  lower  end  of  the  polar  axis,  and  altogether 
it  is  about  the  simplest  and  cheapest  equatorial  mount  of  first 
class  quality  that  can  be  devised  for  carrying  instruments  of 
moderate  length. 

Several  others  are  in  use  at  the  Harvard  Observatory  and  very 
similar  ones  of  a  larger  growth  carry  the  24  inch  Newtonian 
reflector  there  used  for  stellar  photography  and  the  16  inch 
Metcalf  photographic  doublet. 

In  fact  the  open  fork  mount,  which  was  developed  by  the  late 


MOUNTINGS 


117 


Dr.  Common,  is  very  well  suited  to  the  mounting  of  big  reflectors. 
It  was  first  adapted  by  him  to  his  3  ft.  reflector  and  later  used  for 
his  two  5  ft.  mirrors,  and  more  recently  for  the  5  ft.  instrument  at 
Mt.  Wilson,  and  a  good  many  others  of  recent  make.  Dr. 
Common  in  order  to  secure  the  easiest  possible  motion  in  R.  A. 
devised  the  plan  of  floating  most  of  the  weight  assumed  by  the 
polar  axis  in  mercury. 

Figure  82  is,  diagrammatically,  this  fork  mount  as  worked  out 
by  Ritchey  for  the  5'  Mt.  Wilson  reflector.  Here  A  is  the  lattice 
tube,  B  the  polar  axis,  C  the  fork  and  D  the  hollow  steel  drum 


Fig.  84. — The  60-inch  as  Cassegrain- 
ian,  F  =  80'. 


85.— The    60-inch 
Cassegrainian,  F  = 


which  floats  the  axis  in  the  mercury  trough  E.  The  great  mirror 
is  here  shown  worked  as  a  simple  Newtonian  of  25  ft.  focal  length. 
As  a  matter  of  fact  it  is  used  much  of  the  time  as  a  Cassegranian. 

To  this  end  the  upper  section  of  tube  carrying  the  oblique 
mirror  is  removed  and  a  shorter  tube  carrying  any  one  of  three 
hyperboloidal  mirrors  is  put  in  its  place.  Fig.  83  is  the  normal 
arrangement  for  visual  or  photographic  work  on  the  long  focus, 
100  ft.  The  dotted  lines  show  the  path  of  the  rays  and  it  will  be 
noticed  that  the  great  mirror  is  not  perforated  as  in  the  usual 
Cassegrainian  construction,  but  that  the  rays  are  brought  out  by 
a  diagonal  flat. 

Figure  84  is  a  similar  arrangement  used  for  stellar  spectroscopy 
with  a  small  flat  and  an  equivalent  focus  of  80  ft.  In  Fig. 
85  a  radically  different  scheme  is  carried  out.     The  hyperboloidal 


118  THE  TELESCOPE 

mirror  now  used  gives  an  equivalent  focus  of  150  ft.,  and  the  auxil- 
iary flat  is  arranged  to  turn  on  an  axis  parallel  to  the  declination 
axis  so  as  to  send  the  reflected  beam  down  the  hollow  polar  axis 
into  a  spectrograph  vault  below  the  southern  end  of  the  axis. 
Obviously  one  cannot  work  near  the  pole  with  this  arrangement 
but  only  through  some  75°  as  indicated  by  the  dotted  lines.  The 
fork  mount  is  not  at  all  universal  for  reflectors,  as  has  already  been 
seen,  and  Cassegrainiaus  of  moderate  size  are  very  commonly 
mounted  exactly  like  refractors. 

We  now  come  to  a  group  of  mounts  which  have  in  common  the 
fundamental  idea  of  a  fixed  eyepiece,  and  incidentally  better 
protection  of  the  observer  against  the  rigors  of  long  winter  nights 
when  the  seeing  may  be  at  its  best  but  the  efficiency  of  the 
observer  is  greatly  diminished  by  discomfort.  Some  of  the 
arrangements  are  also  of  value  in  facilitating  the  use  of  long  focus 
objectives  and  mirrors  and  escaping  the  cost  of  the  large  domes 
which  otherwise  would  be  required. 

Perhaps  the  earliest  example  of  the  class  is  found  in  Caroline 
Herschel's  comet  seeker,  shown  in  Fig.  86.  This  was  a  New- 
tonian reflector  of  about  6  inches  aperture  mounted  in  a  fashion 
that  is  almost  self  explanatory.  It  was,  like  all  Herschel's 
telescopes,  an  alt-azimuth  but  instead  of  being  pivoted  in  altitude 
about  the  mirror  or  the  center  of  gravity  of  the  whole  tube,  it 
was  pivoted  on  the  eyepiece  location  and  the  tube  was  counter- 
balanced as  shown  so  that  it  could  be  very  easily  adjusted  in 
altitude  while  the  whole  frame  turned  in  azimuth  about  a 
vertical  post. 

Thus  the  observer  could  stand  or  sit  at  ease  sweeping  in  a 
vertical  circle,  and  merely  had  to  move  around  the  post  as  the 
azimuth  was  changed.  The  arrangement  is  not  without  advan- 
tages, and  was  many  years  later  adopted  with  modifications 
of  detail  by  Dr.  J.  W.  Draper  for  the  famous  instrument  with 
which  he  advanced  so  notably  the  art  of  celestial  photography. 

The  same  fundamental  idea  of  freeing  the  observer  from  con- 
tinual climbing  about  to  reach  the  eyepiece  has  been  carried  out 
in  various  equatorially  mounted  comet  seekers.  A  very  good 
example  of  the  type  is  a  big  comet  seeker  by  Zeiss,  shown  in 
Fig.  87.  The  fundamental  principle  is  that  the  ocular  is  at  the 
intersection  of  the  polar  and  declination  axis,  the  telescope  tube 
being  overhung  well  beyond  the  north  end  of  the  former  and 
counterbalanced  on  the  latter.     The  observer  can  therefore  sit 


MOUNTINGS 


119 


in  his  swivel  chair  and  without  stirring  from  it  sweep  rapidly 
over  a  very  wide  expanse  of  sky. 

This  particular  instrument  is  probably  the  largest  of  regular 
comet  seekers,  8  inches  in  clear  aperture  and  523^  inches  focal 
length  with  a  triple  objective  to  ensure  the  necessary  corrections 
in  using  so  great  a  relative  aperture.  In 
this  figure  1  is  the  base  with  corrections 
in  altitude  and  azimuth,  2  the  counter- 
poise of  the  whole  telescope  on  its  base, 
3  the  polar  axis  and  R.  A.  circle,  4  the 
overhung  declination  axis  and  its  circle, 
5  the  counterpoise  in  declination,  6  the 
polar  counterpoise,  and  7  the  main  tele- 
scope tube.  The  handwheel  shown 
merely  operates  the  gear  for  revolving 
the  dome  without  leaving  the  observing 
chair. 

The  next  step  beyond  the  eyepiece 
fixed  in  general  position  is  so  to  locate 
it  that  the  observer  can  be  thoroughly 
protected  without  including  the  optical 
parts  of  the  telescope  in  such  wise  as  to 
injure  their  performance. 

One  cannot  successfully  observe  through  an  open  window  on 
account  of  the  air  currents  due  to  temperature  differences,  and 
in  an  observatory  dome,  unheated  as  it  is,  must  wait  after  the 
shutter  is  opened  until  the  temperature  is  fairly  steadied. 

Except  for  these  comet  seekers  practically  all  of  the  class 
make  use  of  one  or  two  auxiliary  reflections  to  bring  the  image 
into  the  required  direction,  and  in  general  the  field  of  possible 
view  is  somewhat  curtailed  by  the  mounting.  This  is  less  of  a 
disadvantage  than  it  would  appear  at  first  thought,  for,  to  begin 
with,  observations  within  20°  of  the  horizon  or  thereabouts  are 
generally  unsatisfactory,  and  the  advantages  of  a  stable  and 
convenient  long  focus  instrument  are  so  notable  as  for  many 
purposes  quite  to  outweigh  some  loss  of  sky-space. 

The  simplest  of  the  fixed  eyepiece  group  is  the  polar  telescope 
of  which  the  rudiments  are  well  shown  in  Fig.  88,  a  mount 
described  by  Sir  Howard  Grubb  in  1880,  and  an  example  of  which 
was  installed  a  little  later  in  the  Crawford  Observatory  in  Cork. 
Here  the  polar  axis  A  is  the  main  tube  of  the  telescope,  and  in 


Fig.  86. — Caroline  Her- 
schel's  Comet  Seeker. 


120 


THE  TELESCOPE 


front  of  the  objective  B,  is  held  in  a  fork  the  dedination  cradle 
and  mirror  C,  by  which  any  object  within  a  wide  sweep  of  declina- 
tion can  be  brought  into  the  field  and  held  there  by  hand  or 
clockwork  through  rotating  the  polar  tube. 

Looked  at  from  another  slant  it  is  a  polar  heliostat,  of  which  the 
telescope  forms  the  driving  axis  in  R.  A.     The  whole  mount  was 


Fig.  87. —  Mounting  of  Large  Comet  Seeker. 

a  substantial  casting  on  wheels  which  ran  on  a  pair  of  rails. 
For  use  the  instrument  was  rolled  to  a  specially  arranged  window 
and  through  it  until  over  its  regular  bearings  on  a  pier  just 
outside. 

A  few  turns  of  the  wheel  D  lowered  it  upon  these,  and  the  back 
of  the  frame  then  closed  the  opening  in  the  wall  leaving  the 
instrument  in  the  open,  and  the  eye  end  inside  the  room.  The 
example  first  built  was  of  only  4  inches  aperture  but  proved  its 
case  admirably  as  a  most  useful  and  convenient  instrument. 

This  mount  with  various  others  of  the  fixed  eyepiece  class 
may  be  regarded  as  derived  from  the  horizontal  photoheliographs 


MOUNTINGS 


121 


used  at  the  1874  transit  of  Venus  and  subsequently  at  many 
total  solar  eclipses.  Given  an  equatorially  mounted  heliostat 
like  Fig.  81  and  it  is  evident  that  the  beam  from  it  may  be  turned 
into  a  horizontal  telescope  placed  in  the  meridian,  (or  for  that 
matter  in  any  convenient  direction)  and  held  there  by  rotation  of 


Fig.  88. — Grubb's  Original  Polar  Telescope. 

the  mirror  in  R.  A.,  but  also  in  declination,  save  in  the  case 
where  the  beam  travels  along  the  extension  of  the  polar  axis. 

For  the  brief  exposure  periods  originally  needed  and  the  slow 
variation  of  the  sun  in  declination  this  heliostatic  telescope  was 
easily  kept  in  adjustment.  The  original  instruments  were  of 
5  inches  aperture  and  40  ft.  focal  length,  and  the  7  inch  heliostat 
mirror  was  provided  with  ordinary  equatorial  clockwork.  Set  up 
with  the  telescope  pointing  along  the  polar  axis  no  continuous 
variation  in  declination  is  needed  and  the  clock  drive  holds  the 
field  steadily,  as  in  any  other  equatorial. 


122 


THE  TELESCOPE 


Figure  89  shows  diagrammatically  the  12  inch  polar  telescope 
used  for  more  than  twenty  years  past  at  the  Harvard  Observatory. 
The  mount  was  designed  by  Mr.  W.  P.  Gerrish  of  the  Harvard 
staff  and  contains  many  ingenious  features.  Unlike  Fig.  88  this 
is  a  fixed  mount,  with  the  eye-end  comfortably  housed  in  a  room 
on  the  second  floor  of  the  main  observatory  building,  and  the 
lower  bearing  on  a  substantial  pier  to  the  southward. 

In  the  figure,  A  is  the  eye  end,  B  the  main  tube  with  the 


Fig.  89. — Diagram  of  Gerrish  Polar  Telescope. 


objective  at  its  lower  end  and  prolonged  by  a  fork  supported  by 
the  bearing  C  and  D  is  the  declination  mirror  sending  the  beam 
upward.  The  whole  is  rotated  in  R.  A.  by  an  electric  clock  drive, 
and  all  the  necessary  adjustments  are  made  from  the  eye  end, 

A  view  of  the  exterior  is  shown  in  Fig.  90,  with  the  mirror  and 
objective  uncovered.  The  rocking  arm  at  the  objective  end, 
operated  by  a  small  winch  beside  the  ocular,  swings  clear  both 
mirror  and  objective  caps  in  a  few  seconds,  and  the  telescope  is 
then  ready  for  use.  Its  focal  length  is  16  ft.  10  inches  and  it  gives 
a  sweep  in  declination  of  approximately  80°.  It  gives  excellent 
definition  and  has  proved  a  most  useful  instrument. 

A  second  polar  telescope  was  set  up  at  the  Harvard  Observa- 
tory station  in  Mandeville,  Jamaica,  in  the  autumn  of  1900. 


MOUNTINGS 


123 


This  was  intended  primarily  for  lunar  photography  and  was 
provided  with  a  12  inch  objective  of  135  ft.  4  inches  focal  length 
and  an  18  inch  heliostat  with  electric  clock  drive. 

Inasmuch  as  all  instruments  of  this  class  necessarily  rotate 


Fig.  90. — Gerrish  Polar  Telescope,  Harvard  Observatory. 

the  image  as  the  mirror  turns,  the  tail-piece  of  this  telescope  is 
also  mounted  for  rotation  by  a  similar  drive  so  that  the  image  is 
stationary  on  the  plate  both  in  position  and  orientation.  As 
Mandeville  is  in  N.  lat.  18°  01'  the  telescope  is  conveniently  near 
the  horizontal.  The  observatory  of  Yale  University  has  a  large 
instrument  of  this  class,  of  50  feet  focal  length,  with  a  15-inch 
photographic  objective  and  a  10-inch  visual  guiding  objective 
working  together  from  the  same  heliostat. 


124 


THE  TELESCOPE 


Despite  its  simplicity  and  convenience  the  polar  telescope 
has  an  obvious  defect  in  its  very  modest  sweep  in  declination, 
only  to  be  increased  by  the  use  of  an  exceptionally  large  mirror. 
It  is  not  therefore  remarkable  that  the  first  serious  attempt  at  a 
fixed  eyepiece  instrument  for  general  use  turned  to  a  different 
construction  even  at  the  cost  of  an  additional  reflection. 

This  was  the  equatorial  coude  devised  by  M.  Loewy  of  the 
Paris  Observatory  in   1882.     (Fig.   91.)     In  the  diagram  A  is 

the  main  tube  which  forms 
the  polar  axis,  and  B  the  eye 
end  under  shelter,  with  all 
accessories  at  the  observers 
hand.  But  the  tube  is  broken 
by  the  box  casing  C  con- 
taining a  mirror  rigidly  sup- 
ported at  45°  to  the  axis  of 
the  main  tube  and  of  the  side 
tube  D,  which  is  counterbal- 
anced and  is  in  effect  a  hollow 
declination  axis  carrying  the 
objective  E  at  its  outer  end. 

In  lieu  of  the  telescope  tube 
usually  carried  on  this  decli- 
nation axis  we  have  the  45°  mirror,  F,  turning  in  a  sleeve  concentric 
with  the  objective,  which,  having  a  lateral  aperture,  virtually 
gives  the  objectives  a  full  sweep  in  declination,  save  as  the  upper 
pier  cuts  it  oS.  The  whole  instrument  is  clock  driven  in  R.  A., 
and  has  the  usual  circles  and  slow  motions  all  handily  manipu- 
lated from  the  eye  end. 

The  equatorial  coude  is  undeniably  complicated  and  costly, 
but  as  constructed  by  Henry  Freres  it  actually  performs  admir- 
ably even  under  severe  tests,  and  has  been  several  times  dupli- 
cated in  French  observatories.  The  first  coude  erected  was  of 
lO)-^  inches  aperture  and  was  soon  followed  by  one  of  23.6  inches 
aperture  and  59  ft.  focus,  which  is  the  largest  yet  built. 

Still  another  mounting  suggestive  of  both  the  polar  telescope 
and  the  coude  is  due  to  Sir  Howard  Grubb,  Fig.  92.  Here  as  in 
the  coude  the  upper  part  of  the  polar  axis.  A,  is  the  telescope  tube 
which  leads  into  a  solid  casing  B,  about  which  a  substantial 
fork,  C,  is  pivoted.  This  fork  is  the  extension  of  the  side  tube  D, 
which  carries  the  objective,  and  thus  has  free  swing  in  declina- 


FiG.  91. — Diagram  of  Equatorial  Coude. 


MOUNTINGS 


125 


tion  through  an  angle  Hmited  by  the  roof  of  the  observing  room 
above,  and  the  proximity  of  the  horizon  below. 

Its  useful  swing,  as  in  the  polar  telescope,  is  limited  by  the 
dimensions  of  the  mirror  E,  which  receives  the  cone  of  rays  from 
the  objective  and  turn  it  up  the  polar  tube  to  the  eye-piece.  This 
mirror  is  geared  to  turn  at  half  the  rate  of  the  tube  D  so  that  the 
angle  D  E  A  is  continually  bisected. 


Fig.  92. — Grubb  Modified  Coude. 

In  point  of  fact  the  sole  gain  in  this  construction  is  the  reduc- 
tion in  the  size  of  mirror  required,  by  reason  of  the  diminished 
size  of  the  cone  of  rays  when  it  reaches  the  mirror.  The  plan 
has  been  very  successfully  worked  out  in  the  fine  astrographic 
telescope  of  the  Cambridge  Observatory  of  123^^  inches  aperture 
and  19.3  ft.  focal  length. 

As  in  the  other  instruments  of  this  general  class  the  adjust- 
ments are  all  conveniently  made  from  the  eye  end.  The  Cam- 
bridge instrument  has  a  triple  photo-visual  objective  of  the  form 
designed  by  Mr.  H.  D.  Taylor  and  the  side  tube,  when  not  in  use, 
is  turned  down  to  the  horizontal  and  covered  in  by  a  low  wheeled 
housing  carried  on  a  track.  The  sky  space  covered  is  from  15° 
above  the  pole  to  near  the  horizontal. 

It  is  obvious  that  various  polar  and  coude  forms  of  reflector  are 


126 


THE  TELESCOPE 


quite  practicable  and  indeed  one  such  arrangement  is  shown  in. 
connection  with  the  60  inch  Mt.  Wilson  reflector,  but  we  are  here 
concerned  only  with  the  chief  types  of  mounting  which  have 
actually  proved  their  usefulness.  None  of  the  arrangements 
which  require  the  use  of  additional  large  reflecting  surfaces  are 
exempt  from  danger  of  impaired  definition.  Only  superlatively 
fine  workmanship  and  skill  in  mounting  can  save  them  from  dis- 
tortion and  astigmatism  due  to  flexure  and  warping  of  the  mirrors, 
and  such  troubles  have  not  infrequently  been  encountered. 

To  a  somewhat  variant  type  belong  several  valuable  construc- 
tions which  utilize  in  the  auxiliary  reflecting  system  the  coelostat 
rather  than  the  polar  heliostat  or  its  equivalent.     The  coelostat 


Fig.  93. — Diagram  of  Snow  Horizontal  Telescope. 


is  simply  a  plane  mirror  mounted  with  its  plane  fixed  in  that  of  a 
polar  axis  which  rotates  once  in  48  hours,  i.e.,  at  half  the  apparent 
rate  of  the  stars. 

A  telescope  pointed  at  such  a  mirror  will  hold  the  stars  motion- 
less in  its  field  as  if  the  firmament  were  halted  d  la  Joshua.  But 
if  a  change  of  view  is  wanted  the  telescope  must  be  shifted  in 
altitude  or  azimuth  or  both.  This  is  altogether  inconvenient, 
so  that  as  a  matter  of  practice  a  second  plane  mirror  is  used  to 
turn  the  steady  beam  from  the  coelostat  into  any  desired 
direction. 

By  thus  shifting  the  mirror  instead  of  the  telescope,  the  latter 
can  be  permanently  fixed  in  the  most  convenient  location,  at  the 
cost  of  some  added  expense  and  loss  of  light.  Further,  the  image 
does  not  rotate  as  in  case  of  the  polar  heliostat,  which  is  often  an 
advantage. 

An  admirable  type  of  the  fixed  telescope  thus  constituted  is 
the  Snow  telescope  at  Mt.  Wilson  (Cont.  from  the  Solar  Obs. 


MOUNTINGS  127 

#2,  Hale).  Fig.  93  from  this  paper  shows  the  equipment  in 
plan  and  elevation.  The  topography  of  the  mountain  top  made 
it  desirable  to  lay  out  the  axis  of  the  building  15°  E.  of  N.  and 
sloping  downward  5°  toward  the  N. 

At  the  right  hand  end  of  the  figure  is  shown  the  coelostat 
pier,  29  ft.  high  at  its  Send.  This  pier  carries  the  coelostat  mirror 
proper,  30  inches  in  diameter,  on  rails  a  a  accurately  E.  and  W.  to 
allow  for  sliding  the  instrument  so  that  its  field  may  clear  the 
secondary  mirror  of  24  inches  diameter  which  is  on  an  alt-azimuth 
fork  mounting  and  also  slides  on  rails  h  b. 

The  telescope  here  is  a  pair  of  parabolic  mirrors  each  of  24  inches 
aperture  and  of  60  ft.  and  145  ft.  focus  respectively.  The  beam 
from  the  secondary  coelostat  mirror  passes  first  through  the 
spectrographic  laboratory  shown  to  the  left  of  the  main  pier,  and 
in  through  a  long  and  narrow  shelter  house  to  one  of  these  mirrors ; 
the  one  of  longest  focus  on  longitudinal  focussing  rails  e  e,  the 
other  on  similar  rails  c  c,  with  provision  for  sliding  sidewise  at  d 
to  clear  the  way  for  the  longer  beam. 

The  ocular  end  of  this  remarkable  telescope  is  the  spectro- 
graphic laboratory  where  the  beam  can  be  turned  into  the 
permanently  mounted  instruments,  for  the  details  of  which  the 
original  paper  should  be  consulted.  The  purpose  of  this  brief 
description  is  merely  to  show  the  beautiful  facility  with  which 
a  coelostatic  telescope  may  be  adapted  to  astrophysical  work. 
Obviously  an  objective  could  be  put  in  the  coelostat  beam  for  any 
purpose  for  which  it  might  be  desirable. 

Such  in  fact  is  the  arrangement  of  the  tower  telescopes  at 
the  Mt.  Wilson  Observatory.  In  these  instruments  we  have  the 
ordinary  coelostat  arrangement  turned  on  end  for  the  sake  of 
getting  the  chief  optical  parts  well  above  the  ground  where, 
removed  from  the  heated  surface,  the  definition  is  generally 
improved.  To  be  sure  the  focus  is  at  or  near  the  ground  level,  but 
the  upward  air  currents  cause  much  less  disturbance  than  the 
crosswise  ones  in  the  Snow  telescope. 

The  head  of  the  first  tower  telescope  is  shown  in  Fig.  94.* 
A  is  the  coelostat  mirror  proper  17  inches  in  diameter  and  12 
inches  thick,  B  the  secondary  mirror  12^^  inches  in  the  shorter 
axis  of  the  ellipse,  223^^  inches  in  the  longer,  and  also  12  inches 

*  Contributions  from  the  Solar  Obs.  #23,  Hale,  which  should  be  seen  for 
details. 


128 


THE  TELESCOPE 


thick.     C  is  the  12  inch  objective  of  60  ft.  focus,  and  D  the  focuss- 
ing gear  worked  by  a  steel  ribbon  from  below. 

This   instrument   being   for   solar   research    the   mirrors   are 


\    \. 


Fig.  94. — Head  of  60-inch  Tower  Telescope. 


arranged  for  convenient  working  with  the  sun  fairly  low  on  either 
horizon  where  the  definition  is  at  its  best,  and  can  be  shifted 
accordingly,  to  the  same  end  as  in  the  Snow  telescope.  There 
is  also  provision  for  shifting  the  objective  laterally  at  a  uniform 


MOUNTINGS 


129 


rate  from  below,  to  provide  for  the  use  of  the  apparatus  as 
spectro-heUograph . 

The  tower  is  of  the  windmill  type  and  proved  to  be  fairly 
steady  in  spite  of  its  height,  high  winds  being  rare  on  Mt.  Wilson. 
The  great  thickness  of  the  mirrors  in  the  effort  to  escape 
distortion  deserves  notice.     They  actually  proved  to  be  too 


^Parabolic 
Mirror 


Sf/ow/^/G^    Vertical   Sbctioiw    thro 

Poi-AR      AX/S        AAJO     ARRANGEAte-HT 
O^       OPT/CAl.       PAftTS  • 


Fig.  95. — Porter's  Polar  Reflector. 


thick  to  give  thermal  conductivity  sufficient  to  prevent  distortion. 
In  the  later  150'  tower  telescope  the  mirrors  are  relatively  less 
thick,  and  a  very  interesting  modification  has  been  introduced 
in  the  tower,  in  that  it  consists  of  a  lattice  member  for  member 
within  another  exterior  lattice,  so  that  the  open  structure  is 
retained,  while  each  member  that  supports  the  optical  parts  is 


10 


130 


THE  TELESCOPE 


shielded  from  the  wind  and  sudden  temperature  change  by  its 
corresponding  outer  sheath. 

Still  another  form  of  mounting  to  give  the  observer  access 
to  a  fixed  eyepiece  under  shelter  is  found  in  the  ingenious  polar 
reflector  by  Mr.  Russell  W.  Porter  of  which  an  example  with  main 
mirror  of  16  inches  diameter  and  15  ft.  6  inches  focal  length  was 
erected  by  him  a  few  years  ago.  Fig.  95  is  entirely  descriptive 
of  the  arrangement  which  from  Mr.  Porter's  account  seems  to 
have  worked  extremely  well.  The  chief  difficulty  encountered 
was  condensation  of  moisture  on  the  mirrors,  which  in  some 
climates  is  very  difficult  to  prevent. 


Fig.  96. — Diagram  of  Hartness  Turret  Telescope. 


It  is  interesting  to  note  that  Mr.  Porter's  first  plan  was  to  use 
the  instrument  as  a  Herschelian  with  its  focus  thrown  below  the 
siderostat  at  F' ,  but  the  tilting  of  the  mirror,  which  was  worked 
at  F/11.6,  produced  excessive  astigmatism  of  the  images,  and  the 
plan  was  abandoned  in  favor  of  the  Newtonian  form  shown  in 
the  figure.  At  F/25  or  thereabouts  the  earlier  scheme  would 
probably  have  succeeded  well. 

Still  another  fixed  eyepiece  telescope  of  daring  and  successful 
design  is  the  turret  telescope  of  the  Hon.  J.  E.  Hartness  of  which 
the  inventor  erected  a  fine  example  of  10  inch  aperture  at  Spring- 
field, Vermont.  The  telescope  is  in  this  case  a  refractor,  and  the 
feature  of  the  mount  is  that  the  polar  axis  is  expanded  into  a 
turret  within  which  the  observer  sits  comfortably,  looking  into 
the  ocular  which  lies  in  the  divided  declination  axis  and  is  sup- 
plied from  a  reflecting  prism  in  the  main  beam  from  the  objective 


MOUNTINGS  131 

Figure  96  shows  a  diagram  of  the  mount  and  observatory. 
Here  a  is  the  polar  turret,  bb  the  bearings  of  the  declination 
axis,  c  the  main  tube,  d  its  support,  and  e  the  ocular  end.  Opti- 
cally the  telescope  is  merely  an  ordinary  refractor  used  with  a 
right  angled  prism  a  little  larger  and  further  up  the  tube  than 
usual.  The  turret  is  entered  in  this  instance  from  below,  through 
a  tunnel  from  the  inventor's  residence.  The  telescope  as  shown 
in  Fig.  96  has  a  10  inch  Brashear  objective  of  fine  optical  quality, 
and  the  light  is  turned  into  the  ocular  tube  by  a  right  angled 
prism  only  2%  inches  in  the  face.  This  is  an  entirely  practicable 
size  for  a  reflecting  prism  and  the  light  lost  is  not  materially 
in  excess  of  that  lost  in  the  ordinary  "star  diagonal"  so  necessary 
for  the  observation  of  stars  near  the  zenith  in  an  ordinary  equa- 
torial. The  only  obvious  difficulty  of  the  construction  is  the 
support  of  the  very  large  polar  axis.  Being  an  accomplished 
mechanical  engineer,  Mr.  Hartness  worked  out  the  details  of 
this  design  very  successfully  although  the  moving  parts  weighed 
some  2  tons.  The  ocular  is  not  absolutely  fixed  with  reference 
to  the  observer  but  is  always  conveniently  placed,  and  the  per- 
formance of  the  instrument  is  reported  as  excellent  in  every 
respect,  while  the  sheltering  of  the  observer  from  the  rigors  of 
a  Vermont  winter  is  altogether  admirable.  Figure  97  shows  the 
complete  observatory  as  it  stands.  Obviously  the  higher  the 
latitude  the  easier  is  this  particular  construction,  which  lends 
itself  readily  to  large  instruments  and  has  the  additional  advan- 
tage of  freeing  the  observer  from  the  insect  pests  which  are 
extremely  troublesome  in  warm  weather  over  a  large  part  of 
the  world. 

This  running  account  of  mountings  makes  no  claim  at  com- 
pleteness. It  merely  shows  the  devices  in  common  use  and  some 
which  point  the  way  to  further  progress.  The  main  require- 
ments in  a  mount  are  steadiness,  and  smoothness  of  motion. 
Even  an  alt-azimuth  mount  with  its  need  of  two  motions,  if 
smooth  working  and  steady,  is  preferable  to  a  shaky  and  jerky 
equatorial. 

Remember  that  the  Herschels  did  immortal  work  without 
equatorial  mountings,  and  used  high  powers  at  that.  A  clock 
driven  equatorial  is  a  great  convenience  and  practically  indispen- 
sable for  the  photographic  work  that  makes  so  large  a  part  of 
modern  astronomy,  but  for  eye  observations  one  gets  on  very 
fairly  without  the  clock. 


132 


THE  TELESCOPE 


Circles  are  a  necessity  in  all  but  the  small  telescopes  used  on 
portable  tripods,  otherwise  much  time  will  be  wasted  in  finding. 


In  any  event  do  not  skimp  on  the  finder,  which  should  be  of 
ample  aperture  and  wide  field,  say  ^^  the  aperture  of  the  main 


MOUNTINGS  133 

objective,  and  3°  to  5°  in  field.  Superior  definition  is  needless, 
light,  and  sky  room  enough  to  locate  objects  quickly  being  the 
fundamental  requisites. 

As  a  final  word  see  that  all  the  adjustments  are  within  easy 
reach  from  the  eyepiece,  since  an  object  once  lost  from  a  high 
power  ocular  often  proves  troublesome  to  locate  again. 

REFERENCES 

Chambers'  Astronomy,  Vol.  II. 

F.  L.  O.  Wadsworth:  A'p.  J.,  5,  132.  Ranyard's  mounts  for  reflectors. 
G..W.  Ritchey:  A-p.  J.,  5,  143.     Supporting  large  specula. 

G.  E.  Hale:  Cont.  Solar  Obs.  #  2.  The  "Snow"  horizontal  telescope. 
G.  E.  Hale:  Cont.  Solar  Obs.  #  23.     The  60  ft.  tower  telescope. 

J.  W.  Draper:  Smithsonian  Con trib.  34.  Mounting  of  his  large  reflector. 
G.  W.  Ritchey:  Smithsonian  Contrib.  35.     Mounting  of  the  Mt.  Wilson 

60  inch  reflector. 
Sir  H.  Grubb:  Tr.  Roy.  Dublin  Soc.  Ser.  2.  3.     Polar  Telescopes. 
Sir  R.  S.  Ball:  M.  N.  59,  152.     Photographic  polar  telescope. 
A.  A.  Common:  Mem.  R.  A.  S.,  46,  173.     Mounting  of  his  3  ft.  reflector. 
R.  W.  Porter:  Pop.  Ast.,  24,  308.     Polar  reflecting  telescope. 
James  Hartness:  Trans.  A.  S.  M.  E.,  1911,  Turret  Telescope. 
Sir  David  Gill:  Enc.  Brit.,  11th  Ed.  Telescope.     Admirable  summary  of 

mounts. 


CHAPTER  VI 
EYE  PIECES 

The  eyepiece  of  a  telescope  is  merely  an  instrument  for 
magnifying  the  image  produced  by  the  objective  or  mirror.  If 
one  looks  through  a  telescope  without  its  eyepiece,  drawing  the 
eye  back  from  the  focus  to  its  ordinary  distance  of  distinct 
vision,  the  image  is  clearly  seen  as  if  suspended  in  air,  or  it  can  be 
received  on  a  bit  of  ground  glass. 

It  appears  larger  or  smaller  than  the  object  seen  by  the  naked 
eye,  in  proportion  as  the  focal  length  of  the  objective  is  larger 
or  smaller  than  the  distance  to  which  the  eye  has  to  drop  back 
to  see  the  image  clearly. 

This  real  image,  the  quality  of  which  depends  on  the 
exactness  of  correction  of  the  objective  or  mirror,  is  then  to  be 
magnified  so  much  as  may  be  desirable,  by  the  eyepiece  of  the 
instrument.  In  broad  terms,  then,  the  eyepiece  is  a  simple 
microscope  applied  to  the  image  of  an  object  instead  of  the 
object  itself. 

And  looking  at  the  matter  in  the  simplest  way  the  magnifying 
power  of  any  simple  lens  depends  on  the  focal  length  of  that  lens 
compared  with  the  ordinary  seeing  distance  of  the  eye.  If  this 
be  taken  at  10  inches  as  it  often  conventionally  is,  then  a  lens  of 
1  inch  focus  brings  clear  vision  down  to  an  inch  from  the  object, 
increases  the  apparent  angle  covered  by  the  object  10  times  and 
hence  gives  a  magnifying  power  of  10. 

But  if  the  objective  has  a  focal  length  of  100  inches  the  image, 
as  we  have  just  seen,  is  already  magnified  10  times  as  the  naked 
eye  sees  it,  hence  with  an  objective  of  100  inches  focus  and  a  1 
inch  eyepiece  the  total  magnification  is  100  diameters.  And  this 
expresses  the  general  law,  for  if  we  took  the  normal  seeing  distance 
of  the  naked  eye  at  some  other  value  than  10  inches,  say  123^^ 
inches  then  we  should  have  to  reckon  the  image  as  magnified  by 
8  times  so  far  as  the  objective  inches  is  concerned,  but  12}^ 
times  due  to  the  1  inch  eyepiece,  and  so  forth.  Thus  the 
magnifying  power  of  any  eyepiece  is  F/f  where  F  is  the  focal 

134 


EYEPIECES  135 

length  of  the  objective  or  mirror  and  f  that  of  the  eyepiece. 
The  focal  distance  of  the  eye  quite  drops  out  of  the  reckoning. 

All  these  facts  appear  very  quickly  if  one  explores  the  image 
from  an  objective  with  a  slip  of  ground  glass  and  a  pocket  lens. 
An  ordinary  camera  tells  the  same  story.  A  distant  object  which 
covers  1°  will  cover  on  the  ground  glass  1°  reckoned  on  a  radius 
equal  to  the  focal  length  of  the  lens.  If  this  is  equal  to  the  ordi- 
nary distance  of  clear  vision,  an  eye  at  the  same  distance  will  see 
the  image  (or  the  distant  object)  covering  the  same  1°. 

The  geometry  of  the  situation  is  as  follows:  Let  o  Fig.  5,  Chap. 
I,  be  the  objective.  This  lens,  as  in  an  ordinary  camera,  forms  an 
inverted  image  of  an  object  A  B  at  its  focus,  as  at  a  h,  and  for 
anj^  point,  as  a,  of  the  image  there  is  a  corresponding  point  of  the 
object  lying  on  the  straight  line  from  A  to  that  point  through  the 
center,  c,  of  the  objective. 

A  pair  of  rays  1,  2,  diverging  from  the  object  point  A  pass 
through  rim  and  center  of  o  respectively  and  meet  in  A.  After 
crossing  at  this  point  they  fall  on  the  eye  lens  e,  and  if  a  is  nearly 
in  the  principal  focus  of  e,  the  rays  1  and  2  will  emerge  sub- 
stantially parallel  so  that  the  eye  will  unite  them  to  form  a  clear 
image. 

Now  if  F  is  the  focal  length  of  o,  and  f  that  of  a,  the  object 
forming  the  image  subtends  at  the  center  of  the  objective,  o,  an 
angle  A  cB,  and  for  a  distant  object  this  will  be  sensibly  the  angle 
under  which  the  eye  sees  the  same  object. 

If  L  is  the  half  linear  dimension  of  the  image,  the  eye  sees  half 
the  object  covering  the  angle  whose  tangent  is  L/F.  Similarly 
half  the  image  ab  is  seen  through  the  eye  lens  e  as  covering  a  half 
angle  whose  tangent  is  L/f.  Since  the  magnifying  power  of  the 
combination,  m,  is  directly  as  the  ratio  of  increase  in  this  tangent 
of  the  visual  angle,  which  measures  the  image  dimension 

F 

m  =  -p,  as  before 

Further,  as  all  the  light  which  comes  in  parallel  through  the 
whole  opening  of  the  objective  forms  a  single  conical  beam  con- 
centrating into  a  focus  and  then  diverging  to  enter  the  eye  lens, 
the  diameter  of  the  cone  coming  through  the  eye  lens  must  bear 
the  same  relation  to  the  diameter  of  o,  that  f  does  to  F. 

Any  less  diameter  of  e  will  cut  off  part  of  the  emerging  light; 
any  more  will  show  an  emergent  beam  smaller  than  the  eye  lens, 


136  THE  TELESCOPE 

which  is  generally  the  case.  Hence  if  we  call  p  the  diameter  of 
the  bright  pencil  of  light  which  we  see  coming  through  the  eye 
lens  then  for  that  particular  eye  lens, 

0 

m  =  — 
P 
pF        .      .        . 
That  is,  f  =    — ?  which  is  quite  the  easiest  way  of  measuring  the 

focal  length  of  an  eyepiece. 

Point  the  telescope  toward  the  clear  sky,  focusing  for  a  distant 
object  so  that  the  emergent  pencil  is  sharply  defined  at  the 
ocular,  and  then  measure  its  diameter  by  the  help  of  a  fine  scale 
and  a  pocket  lens,  taking  care  that  scale  and  emergent  pencil  are 
simultaneously  in  sharp  focus  and  show  no  parallax  as  the  eye  is 
shifted  a  bit.  This  bright  circle  of  the  emerging  beam  is  actually 
the  projection  by  the  eye  lens  of  the  focal  image  of  the  objective 
aperture. 

This  method  of  measuring  power  is  easy  and  rather  accurate. 
But  it  leads  to  trouble  if  the  measured  diameter  of  the  objective 
is  in  fact  contracted  by  a  stop  anywhere  along  the  path  of  the 
beam,  as  occasionally  happens.  Examine  the  telescope  carefully 
with  reference  to  this  point  before  thus  testing  the  power.* 

The  eye  lens  of  Fig.  5  is  a  simple  double  convex  one,  such  as 
was  used  by  Christopher  Scheiner  and  his  contemporaries. 
With  a  first  class  objective  or  mirror  the  simple  eye  lens  such 
as  is  shown  in  Fig.  98a  is  by  no  means  to  be  despised  even  now. 
Sir  William  Herschel  always  preferred  it  for  high  powers,  and 
speaks  with  evident  contempt  of  observers  who  sacrificed  its 
advantages  to  gain  a  bigger  field  of  view.  Let  us  try  to  fathom 
the  reason  for  his  vigorously  expressed  opinion,  strongly  backed 
up  by  experienced  observers  like  the  late  T.  W.  Webb  and  Mr. 
W.  F.  Denning. 

First  of  all  a  single  lens  saves  about  10%  of  the  light.  Each 
surface  of  glass  through  which  light  passes  transmits  95  to  96  % 
of  that  light,  so  that  a  single  lens  transmits  approximately  90%, 
two  lenses  81%  and  so  on.  This  loss  may  be  enough  to  deter- 
mine the  visibility  of  an  object.  Sir  Wm.  Herschel  found  that 
faint  objects  invisible  with  the  ordinary  two  lens  eyepiece  came 
to  view  with  the  single  lens. 

*  A  more  precise  method,  depending  on  an  actual  measurement  of  the 
angle  subtended  by  the  diameter  of  the  eyepiece  diaphragm  as  seen  through 
the  eye  end  of  the  ocular  and  its  comparison  with  the  same  angular  diameter 
reckoned  from  the  objective,  is  given  by  Schaeberle.     M.  N.  43,     297. 


EYEPIECES 


137 


Probably  the  actual  loss  is  less  serious  than  its  effect  on  seeing 
conditions.  The  loss  is  due  substantially  to  reflection  at  the 
surfaces,  and  the  light  thus  reflected  is  scattered  close  to,  or 
into,  the  eye  and  produces  stray  light  in  the  field  which  injures 
the  contrast  by  which  faint  objects  become  visible. 

In  some  eyepieces  the  form  of  the  surfaces  is  such  that  reflected 
light  is  strongly  concentrated  where  the  eye  sees  it,  forming 
a  "ghost"  quite  bright  enough  greatly  to  interfere  with  the  vision 
of  delicate  contrasts. 

The  single  lens  has  a  very  small  sharp  field,  hardly  10°  in 
angular  extent,  the  image  falling  off  rapidly  in  quality  as  it 
departs  from  the  axis.     If  plano-convex,  as  is  the  eye  lens  of 


Fig.  98. — Simple  Oculars. 

common  two-lens  oculars,  It  works  best  with  the  curved  side  to 
the  eye,  i.e.,  reversed  from  its  usual  position,  the  spherical 
aberration  being  much  less  in  this  position. 

Herschel's  report  of  better  definition  with  a  single  lens  than 
with  an  ordinary  two  lens  ocular  speaks  ill  for  the  quality  of 
the  latter  then  available.  Of  course  the  single  lens  gives  some 
chromatic  aberration,  generally  of  small  account  with  the  narrow 
pencils  of  light  used  in  high  powers. 

A  somewhat  better  form  of  eye  lens  occasionally  used  is  the 
so-called  Coddington  lens,  really  devised  by  Sir  David  Brewster. 
This,  Fig.  986,  is  derived  from  a  glass  sphere  with  a  thick  equatorial 
belt  removed  and  a  groove  cut  down  centrally  leaving  a  diameter 
of  less  than  half  the  radius  of  the  sphere.  The  focus  is,  for  ordi- 
nary crown  glass,  %  the  radius  of  the  sphere,  and  the  field  is  a 
little  improved  over  the  simple  lens,  but  it  falls  off  rather  rapidly, 
with  considerable  color  toward  the  edge. 

The  obvious  step  toward  fuller  correction  of  the  aberrations 
while  retaining  the  advantages  of  the  simple  lens  is  to  make  the 
ocular  achromatic,  like  a  minute  objective,  thus  correcting  at 
once  the  chromatic  and  spherical  aberrations  over  a  reasonably 
large  field.  As  the  components  are  cemented  the  loss  of  light  at 
their  common  surface  is  negligible.     Figure  98c  shows  such  a 


138 


THE  TELESCOPE 


lens.  If  correctly  designed  it  gives  an  admirably  sharp  field  of 
15°  to  20°,  colorless  and  with  very  little  distortion,  and  is  well 
adapted  for  high  powers. 

Still  better  results  in  field  and  orthoscopy  can  be  attained 
by  going  to  a  triple  cemented  lens,  similar  to  the  objective  of 


a 
Fig.  99. 


-Triple  Cemented  Oculars. 


Fig.  57.  Triplets  thus  constituted  are  made  abroad  by  Zeiss, 
Steinheil  and  others,  while  in  this  country  an  admirable  triplet 
designed  by  Professor  Hastings  is  made  by  Bausch  &  Lomb. 

Such  lenses  give  a  beautifully  flat  and  sharp  field  over  an  angle 
of  20°  to  30°,  quite  colorless  and  orthoscopic.     Fig.  99a,  a  form 


Fig.  100. — Path  of  Rays  Through  Huygenian  Ocular. 

used  by  Steinheil,  is  an  excellent  example  of  the  construction 
and  a  most  useful  ocular.  The  late  R.  B.  Tolles  made  such 
triplets,  even  down  to  3^^  inch  focus,  which  gave  admirable 
results. 

A  highly  specialized  form  of  triplet  is  the  so-called  mono- 
centric  of  Steinheil  Fig.  996.  Its  peculiarity  is  less  in  the  fact 
that  all  the  curves„are  struck  from  the  same  center  than  in  the 
great  thickness  of  the  front  flint  and  the  crown,  which,  as  in  some 


EYEPIECES  139 

photographic  lenses,  give  added  facilities  for  flattening  the  field 
and  eliminating  distortion. 

The  monocentric  eyepiece  has  a  high  reputation  for  keen 
definition  and  is  admirably  achromatic  and  orthoscopic.  The 
sharp  field  is  about  32°,  rather  the  largest  given  by  any  of  the 
cemented  combinations.  All  these  optically  single  lenses  are 
quite  free  of  ghosts,  reduce  scattered  light  to  a  minimum,  and 
leave  little  to  be  desired  in  precise  definition.  The  weak  point 
of  the  whole  tribe  is  the  small  field,  which,  despite  Herschel's 
opinion,  is  a  real  disadvantage  for  certain  kinds  of  work  and 
wastes  the  observer's  time  unless  his  facilities  for  close  setting 
are  more  than  usually  good. 

Hence  the  general  use  of  oculars  of  the  two  lens  types,  all 
of  them  giving  relatively  wide  fields,  some  of  them  faultless  also 
in  definition  and  orthoscopy.  The  earliest  form,  Fig.  100,  is 
the  very  useful  and  common  one  used  by  Huygens  and  bearing 
his  name,  though  perhaps  independently  devised  by  Campani  of 
Rome.  Probably  four  out  of  five  astronomical  eyepieces  belong 
to  this  class. 

The  Huygenian  ocular  accomplishes  two  useful  results — 
first,  it  gives  a  wider  sharp  field  than  any  single  lens,  and  second 
it  compensates  the  chromatic  aberration,  which  otherwise  must 
be  removed  by  a  composite  lens.  It  usually  consists  of  a  plano- 
convex lens,  convex  side  toward  the  objective,  which  is  brought 
inside  the  objective  focus  and  forms  an  image  in  the  plane  of  a 
rear  diaphragm,  and  a  similar  eye  lens  of  shorter  focus  by  which 
this  image  is  examined. 

Fig.  100  shows  the  course  of  the  rays — A  being  the  field  lens, 
B  the  diaphragm  and  C  the  eye  lens.  Let  i,  ^,  be  rays  which  are 
incident  near  the  margin  of  A.  Each,  in  passing  through 
the  lens,  is  dispersed,  the  blue  being  more  refracted  than  the 
red.  Both  rays  come  to  a  general  focus  at  B,  and,  crossing, 
diverge  slightly  towards  C. 

But,  on  reaching  C,  ray  1,  that  was  nearer  the  margin  and 
the  more  refracted  because  in  a  zone  of  greater  pitch,  now  falls  on 
C  the  nearer  its  center,  and  is  less  refracted  than  ray  2  which 
strikes  C  nearer  the  rim.  If  the  curvatures  of  A  and  C  are 
properly  related  1  and  2  emerge  from  C  parallel  to  each  other 
and  thus  unite  in  forming  a  distinct  image. 

Now  follow  through  the  two  branches  ofJLmarkedX,  andX, 
the  red  and  violet  components.     Ray  X,,  the  more  refrangible, 


140  THE  TELESCOPE 

strikes  C  nearer  the  center,  and  is  the  less  refracted,  emerging 
from  C  substantially  parallel  with  its  mate  Ir,  hence  blending 
the  red  and  violet  images,  if,  being  of  the  same  glass,  A  and  C 
have  suitably  related  focal  lengths  and  separation. 

As  a  matter  of  fact  the  condition  for  this  chromatic  compensa- 
tion is 


d  = 


where  d  is  the  distance  between  the  lenses  and  f,  f,  their  respec- 
tive focal  lengths.  If  this  condition  of  achromatism  be  combined 
with  that  of  equal  refraction  at  A  and  C,  favorable  to  minimizing 
the  spherical  aberration,  we  find  f  =  3f'  and  d  =  2f'.  This  is 
the  conventional  Huygenian  ocular  with  an  eye  lens  3^^  the  focus 
of  the  field  lens,  spaced  at  double  the  focus  of  the  eye  lens,  with 
the  diaphragm  midway. 

In  practice  the  ratio  of  foci  varies  from  1 : 3  to  1 : 2  or  even 
1:1.5,  the  exact  figure  varying  with  the 
amount  of  over-correction  in  the  objective 
and  under-correction  in  the  eye  that  has  to 
be  dealt  with,  while  the  value  of  d  should 
be  adjusted  by  actual  trial  on  the  telescope 
-^        '  to  obtain  the  best  color  correction  practic- 

FiG.  101a.    Airy  8«#      able.     One  cannot  use  any  chance  ocular 

MittBRzwey  Ocularl. 

and  expect  the  finest  results. 

The  Huygenian  eyepieces  are  of  ten  referred  to  as  "  negative  " 
inasmuch  as  they  cannot  be  used  directly  as  magnifiers,  although 
dealing  effectively  with  an  image  rather  than  an  object.  The 
statement  is  also  often  made  that  they  cannot  be  used  with 
cross  wires.  This  is  incorrect,  for  while  there  is  noticeable  dis- 
tortion toward  the  edge  of  the  wide  field,  to  say  nothing  of  astig- 
matism, in  and  near  the  center  of  the  field  the  situation  is  a 
good  deal  better. 

Central  cross  wires  in  the  plane  of  the  diaphragm  are  entirely 
suitable  for  alignment  of  the  instrument,  and  over  a  moderate 
extent  of  field  the  distortion  is  so  small  that  a  micrometer  scale 
in  the  plane  of  the  diaphragm  gives  very  good  approximate 
measurements,  and  indeed  is  widely  used  in  microscopy. 

It  should  be  noted  that  the  achromatism  of  this  type  of  eye- 
piece is  compensatory  rather  than  real.     One  cannot  at  the  same 


EYEPIECES  141 

time  bring  the  images  of  various  colors  to  the  same  size,  and  also 
to  the  same  plane.  As  failure  in  the  latter  respect  is  compara- 
tively unimportant,  the  Huygenian  eyepiece  is  adjusted  so  far 
to  compensate  the  paths  of  the  various  rays  as  to  bring  the 
colored  images  to  the  same  size,  and  in  point  of  fact  the  result 
is  very  good. 

The  field  of  the  conventional  form  of  Huygenian  ocular  is 
fully  40°,  and  the  definition,  particularly  centrally,  is  very 
excellent.  There  are  no  perceptible  ghosts  produced,  and  while 
some  10%  of  light  is  lost  by  reflection  in  the  extra  lens  it  is 
diffused  in  the  general  field  and  is  damaging  only  as  it  injures 
the  contrast  of  faint  objects.  The  theory  of  the  Huygenian  eye- 
piece was  elaborately  given  by  Littrow,  (Memoirs  R.  A.  S.  Vol.  4, 
p.  599),  wherein  the  somewhat  intricate  geometry  of  the  situation 
is  fully  discussed. 

Various  modifications  of  the  Huygenian  type  have  been  devised 
and  used.  Figure  101a  is  the  Airy  form  devised  as  a  result  of  a 
somewhat  full  mathematical  investigation  by 
Sir  George  Airy,  later  Astronomer  Royal. 
Its  peculiarity  lies  in  the  form  of  the  lenses 
which  preserve  the  usual  3:1  ratio  of  focal 
lengths.  The  field  lens  is  a  positive  meniscus 
with  a  noticeable  amount  of  concavity  in  the 


rear  face  while  the  eye  lens  is  a  "crossed"  swi Mittenzuey 

lens,  the  outer  curvature  being  about  3^^  of  Oculars. 

the  inner  curvature.     The  marginal  field  in  this  ocular  is  a  little 
better  than  in  the  conventional  Huygenian. 

A  commoner  modification  now-a-days  is  the  Mittenzwey 
form.  Fig.  1016.  This  is  usually  made  with  2:1  ratio  of  focal 
lengths,  and  the  field  lens  still  a  meniscus,  but  less  conspicuously 
concave  than  in  the  Airy  form.  The  eye  lens  is  the  usual  plano- 
convex. It  is  widely  used,  especially  abroad,  and  gives  perhaps 
as  large  available  field  as  any  ocular  yet  devised,  approximately 
50°,  with  pretty  good  definition  out  to  the  margin. 

Finally,  we  come  to  the  solid  eyepiece  Fig.  102a,  devised  by 
the  late  R.  B.  ToUes  nearly  three  quarters  of  a  century  ago,  and 
and  often  made  by  him  both  for  telescopes  and  microscopes. 
It  is  practically  a  Huygenian  eyepiece  made  out  of  a  single 
cylinder  of  glass  with  a  curvature  ratio  of  13^^:1  between  the  eye 
and  the  field  lens.  A  groove  is  cut  around  the  long  lens  at  about 
yi  its  length  from  the  vertex  of  the  field  end.     This  serves  as  a 


142 


THE  TELESCOPE 


stop,  reducing  the  diameter  of  the  lens  to  about  one-half  its 
focal  length. 

It  is  in  fact  a  Huygenian  eyepiece  free  from  the  loss  of  light 
in  the  usual  construction.  It  gives  a  wide  field,  more  extensive 
than  in  the  ordinary  form,  with  exquisite  definition.  It  is  really 
a  most  admirable  form  of  eyepiece  which  should  be  used  far 
more  than  is  now  the  case.  The  late  Dr.  Brashear  is  on  record 
as  believing  that  all  negative  eyepieces  less  than  %  inch  focus 
should  be  made  in  this  form. 

So  far  as  the  writer  can  ascertain  the  only  reason  that  it  is  not 
more  used  is  that  it  is  somewhat  more  difficult  to  construct  than 


Fig.  102. — Tolles'  Solid  wsd.  Compensated  Ocular^. 

the  two  lens  form,  for  its  curvatures  and  length  must  be  very 
accurately  adjusted.  It  is  consequently  unpopular  with  the  con- 
structing optician  in  spite  of  its  conspicuous  merits.  It  gives  no 
ghosts,  and  the  faint  reflection  at  the  eye  end  is  widely  spread  so 
that  if  the  exterior  of  the  cylinder  is  well  blackened,  as  it  should 
be,  it  gives  exceptional  freedom  from  stray  light.  Still  another 
variety  of  the  Huygenian  ocular  sometimes  useful  is  analogous  to 
the  compensating  eyepiece  used  in  microscopy.  If,  as  commonly 
is  the  case,  a  telescope  objective  is  over-corrected  for  color  to  cor- 
rect for  the  chromatism  of  the  eye  in  low  powers,  the  high  powers 
show  strong  over  correction,  the  blue  focus  being  longer  than  the 
red,  and  the  blue  image  therefore  the  larger. 

If  now  the  field  lens  of  the  ocular  be  made  of  heavy  flint  glass 
and  the  separation  of  the  lenses  suitably  adjusted,  the  stronger 
refraction  of  the  field  lens  for  the  blue  pulls  up  the  blue  focus  and 
brings  its  image  to  substantially  the  dimensions  of  the  red,  so 
that  the  eye  lens  performs  as  if  there  were  no  overcorrection 
of  the  objective. 

The  writer  has  experimented  with  an  ocular  of  this  sort  as 
shown  in  Fig.  1026  and  finds  that  the  color  correction  is,  as 
might  be  expected,  greatly  improved  over  a  Mittenzwey  ocular 


EYEPIECES 


143 


of  the  same  focus  {^i  inch).  There  would  be  material  advantage 
in  thus  varying  the  ocular  color  correction  to  suit  the  power. 

In   the  Huyghenian  eyepiece  the  equivalent  focal  length  F  is 
given  by, 

where  f  and  f  are  the  focal  lengths  of  the  field  and  eye  lenses 
respectively.  This  assumes  the  normal  spacing,  d,  of  half  the 
sum  of  the  focal  lengths,  not  always  adhered  to  by  constructors. 
The  perfectly  general  case,  as  for  any  two  combined  lenses  is, 

f  +  f  1  -  d 


F  = 


To  obtain  a  flatter  field,  and  particularly  one  free  from  distor- 
tion the  construction  devised  by  Ramsden  is  commonly  used. 


Fig.  103. — Path  of  Rays  Through  Ramsden  Ocular. 

This  consists.  Fig.  103,  of  two  piano  convex  lenses  of  equal  focal 
length,  placed  with  their  plane  faces  outward,  at  a  distance  equal 
to,  or  somewhat  less  than,  their  common  focal  length.  The 
former  spacing  is  the  one  which  gives  the  best  achromatic  com- 
pensation  since    as   before    the    condition   for   achromatism   is 

d  =  >^(f  +  f) 

When  thus  spaced  the  plane  surface  of  the  field  lens  is  exactly 
in  the  focus  of  the  eye  lens,  the  combined  focus  F  is  the  same  as 
that  of  either  lens,  since  as  just  shown  in  any  additive  combina- 
tion of  two  lenses 

ff' 


F  = 


f  -Hf 


and  while  the  field  is  flat  and  colorless,  every  speck  of  dust  on  the 
field  lens  is  offensively  in  view. 


144  THE  TELESCOPE 

It  is  therefore  usual  to  make  this  ocular  in  the  form  suggested 
by  Airy,  in  which  something  of  the  achromatic  correction 
is  sacrificed  to  obviate  this  difficulty,  and  to  obtain  a  better 
balance  of  the  residual  aberrations.  The  path  of  the  rays  is 
shown  in  Fig.  103.  The  lenses  A  and  B  are  of  the  same  focal 
length  but  are  now  spaced  at  ^^  of  this  length  apart. 

The  two  neighboring  rays  1,  2,  coming  through  the  objective 
from  the  distant  object  meet  at  the  objective  focus  in  a  point,  a, 
of  the  image  plane  a  h.  Thence,  diverging,  they  are  so  refracted 
by  A  and  B  as  to  leave  the  latter  substantially  parallel  so  that 
both  appear  to  proceed  from  the  point  c,  of  the  image  plane  c,  d, 
in  the  principal  focus  of  B. 

From  the  ordinary  equation  for  the  combination,  F  =  %  f. 
The  combination  focusses  3^^  f  back  of  the  principal  focus  of  the 
objective,  and  the  position  of  the  eye  is  3^^  F  back  of  the  eye 
lens,  which  is  another  reason  for  shortening  the  lens  spacing. 
At  longer  spacing  the  eye  distance  is  inconveniently  reduced. 

Thus  constituted,  the  Ramsden  ocular,  known  as  ''positive" 
from  its  capability  for  use  as  a  magnifier  of  actual  objects,  gives 
a  good  flat  field  free  from  distortion  over  a  field  of  nearly  35°  and 
at  some  loss  of  definition  a  little  more.  It  is  the  form  most 
commonly  used  for  micrometer  work. 

In  all  optical  instruments  the  aberrations  increase  as  one 
departs  from  the  axis,  so  that  angular  field  is  rather  a  loose  term 
depending  on  the  maximum  aberrations  that  can  be  tolerated.^ 

Of  the  Ramsden  ocular  there  are  many  modifications.  Some- 
times f  and  f  are  made  unequal  or  there  is  departure  from  the 
simple  plano-convex  form.  More  often  the  lenses  are  made 
achromatic,  thus  getting  rid  of  the  very  perceptible  color  in  the 
simpler  form  and  materially  helping  the  definition.  Figure  104a 
shows  such  an  achromatic  ocular  as  made  by  Steinheil.  The 
general  arrangement  is  as  in  the  ordinary  Ramsden,  but  the 
sharp  field  is  slightly  enlarged,  a  good  36°,  and  the  definition  is 
improved  quite  noticeably. 

A  somewhat  analogous  form,  but  considerably  modified  in 

1  The  angular  field  a  is  defined  by 

•y 

tan  \i&  =  ^ 
F 

where  y  is,  numerically,  the  radius  of  the  field  sharp  enough  for  the  pur- 
pose in  hand,  and  F  the  effective  focal  length  of  the  ocular. 


EYEPIECES  145 

detail,  is  the  Kellner  ocular,  Fig.  1046.  It  was  devised  by  an 
optician  of  that  name,  of  Wetzlar,  who  exploited  it  some  three 
quarters  of  a  century  since  in  a  little  brochure  entitled  "Das 
orthoskopische  Okular,"  as  notable  a  blast  of  "hot  air"  as  ever 
came  from  a  modern  publicity  agent. 

As  made  today  the  Kellner  ocular  consists  of  a  field  lens  which 
is  commonly  plano-convex,  piano  side  out,  but  sometimes  crossed 
or  even  equi convex,  combined  with  a  considerably  smaller  eye 
lens  which  is  an  over-corrected  achromatic.  The  focal  length 
of  the  field  lens  is  approximately  J^  F,  that  of  the  eye  lens  ^:3  F, 
separated  by  about  %  F. 

This  ocular  has  its  front  focal  plane  very  near  the  field  lens, 
sometimes  even  within  its  substance,  and  a  rather  short  eye 
distance,  but  it  gives  admirable  definition  and  a  usable  field  of 
very  great  extent,  colorless  and  orthoscopic  to  the  edge.     The 


a  b 

Fig.   104. — Achromatic  and  Kellner  Oculars. 

writer  has  one  of  2%"  focus,  with  an  achromatic  triplet  as  eye 
lens,  which  gives  an  admirable  field  of  quite  50°. 

The  Kellner  is  decidedly  valuable  as  a  wide  field  positive 
ocular,  but  it  has  in  common  with  the  two  just  previously 
described  a  sometimes  unpleasant  ghost  of  bright  objects. 
This  arises  from  light  reflected  from  the  inner  surface  of  the  field 
lens,  and  back  again  by  the  front  surface  to  a  focus.  This  focus 
commonly  lies  not  far  back  of  the  field  lens  and  quite  too  near 
to  the  focus  of  the  eye  lens  for  comfort.  It  should  be  watched 
for  in  going  after  faint  objects  with  oculars  of  the  types  noted. 

A  decidedly  better  form  of  positive  ocular  is  the  modern 
orthoscopic  as  made  by  Steinheil  and  Zeiss,  Fig.  105a.  It 
consists  of  a  triple  achromatic  field  lens,  a  dense  flint  between 
two  crowns,  with  a  plano-convex  eye  lens  of  much  shorter  focus 
(}/s  to  3^^)  almost  in  contact  on  its  convex  side. 

The  field  triplet  is  heavily  over-corrected  for  color,  the  front 

focal  plane  is  nearly  }--^  F  ahead  of  the  front  vertex  of  the  field 

lens,  and  the  eye  distance  is  notably  greater  than  in  the  Kellner. 

The  field  is  above  40°,  beautifully  flat,  sharp,  and  orthoscopic, 
11 


146 


THE  TELESCOPE 


free  of  troublesome  ghosts.  On  the  whole  the  writer  is  inclined 
to  rate  it  as  the  best  of  two-lens  oculars. 

There  should  also  here  be  mentioned  a  very  useful  long  relief 
ocular,  often  used  for  artillery  sights,  and  shown  in  Fig.  105?). 
It  consists  like  Fig.  104a,  of  a  pair  of  achromatic  lenses,  but  they 
are  placed  with  the  crowns  almost  in  contact  and  are  frequently 
used  with  a  simple  piano  covex  field  lens  of  much  longer  focus, 
to  render  the  combination  more  fully  orthoscopic. 

The  field,  especially  with  the  field  lens,  is  wide,  quite  40°  as 
apparent  angle  for  the  whole  instrument,  and  the  eye  distance  is 
roughly  equal  to  the  focal  length.     It  is  a  form  of  ocular  that 


Fig.  105.— Orthoscopic  and  Long  Relief  Oculars. 

might  be  very  advantageously  used  in  finders,  where  one  often 
has  to  assume  uncomfortable  angles  of  view,  and  long  relief  is 
valuable. 

Whatever  the  apparent  angular  field  of  an  ocular  may  be, 
the  real  angular  field  of  view  is  obtained  by  dividing  the  apparent 
field  by  the  magnifying  power.  Thus  the  author's  big  Kellner, 
just  mentioned,  gives  a  power  of  20  with  the  objective  for  which 
it  was  designed,  hence  a  real  field  of  23-^°,  while  a  second,  power 
65,  gives  a  real  field  of  hardly  0°40',  the  apparent  field  in  this  case 
being  a  trifle  over  40°.  There  is  no  escaping  this  relation,  so  that 
high  power  always  implies  small  field. 

The  limit  of  apparent  field  is  due  to  increasing  errors  away 
from  the  axis,  strong  curvature  of  the  field,  and  particularly 
astigmatism  in  the  outer  zones.  The  eye  itself  can  take  in  only 
about  40°  so  that  more  than  this,  while  attainable,  can  only  be 
utilized  by  peering  around  the  marginal  field. 

For  low  powers  the  usable  field  is  helped  out  by  the  accom- 
modation of  the  eye,  but  in  oculars  of  short  focus  the  curvature 
of  field  is  the  limiting  factor.  The  radius  of  curvature  of  the 
image  is,  in  a  single  lens  approximately  %  F,  and  in  the  common 
two  lens  forms  about  ^^  F. 

In  considering  this  matter  Conrady  has  shown  (M.  N.  78 
445)  that  for  a  total  field  of  40°  the  sharpness  of  field  fails  at  a 


EYEPIECES  147 

focal  length  of  about  1  inch  for  normal  power  of  accommodation. 
The  best  achromatic  combinations  reduce  this  limit  to  about 
}/2  inch. 

At  focal  lengths  below  this  the  sharpest  field  is  obtainable 
only  with  reduced  aperture.  There  is  an  interesting  possi- 
bility of  building  an  anastigmatic  ocular  on  the  lines  of  the  mod- 
ern photographic  lens,  which  Conrady  suggests,  but  the  need  of 
wide  field  in  high  powers  is  hardly  pressing  enough  to  stimulate 
research. 

Finally  we  may  pass  to  the  very  simple  adjunct  of  most 
small  telescopes,  the  terrestrial  ocular  which  inverts  the  image 
and  shows  the  landscape  right  side  up.     Whatever  its  exact 


c  B  A 

Fig.  106. — Ordinary  Terrestrial  Ocular. 

form  it  consists  of  an  inverting  system  which  erects  the  inverted 
image  produced  by  the  objective  alone,  and  an  eyepiece  for  view- 
ing this  erected  image.  In  its  common  form  it  is  composed  of 
four  plano-convex  lenses  arranged  as  in  Fig.  106.  Here  A  and  B 
for  the  inverting  pair  and  C  and  D  a  modified  Huygenian  ocular. 
The  image  from  the  objective  is  formed  in  the  front  focus  of  AB 
which  is  practically  an  inverted  ocular,  and  the  erected  image  is 
formed  in  the  usual  way  between  C  and  D. 

The  apparent  field  is  fairly  good,  about  35°,  and  while  slightly 
better  corrections  can  be  gained  by  using  lenses  of  specially 
adjusted  curvatures,  as  Airy  has  shown,  these  are  seldom  applied. 
The  chief  objection  to  this  erecting  system  is  its  length,  some  ten 
times  its  equivalent  focus.  Now  and  then  to  save  light  and  gain 
field,  the  erector  is  a  single  cemented  combination  and  the  ocular 
like  Fig.  99a  or  Fig.  102a.  Fig.  107  shows  a  terrestrial  eyepiece  so 
arranged,  from  an  example  by  the  late  R.  B.  Tolles.  When  care- 
fully designed  an  apparent  field  of  40°  or  more  can  be  secured, 
with  great  brilliancy,  and  the  length  of  the  erecting  system  is 
moderate. 

Very  much  akin  in  principle  is  the  eyepiece  microscope,  such 
as  is  made  bj^  Zeiss  to  give  variable  power  and  a  convenient 
position  of  the  eye  in  connection  with  filar  micrometers,  Fig.  108. 


148 


THE  TELESCOPE 


It  is  prQvided  with  a  focussing  collar  and  its  draw  tube  allows 
varying  power  just  as  in  case  of  an  ordinary  microscope.  In  fact 
eyepiece  microscopes  have  long  been  now  and  then  used  to 
advantage  for  high  powers.     They  are  easier  on  the  eye,  and  give 


■^ 


Fig.  107.— Tolles  Triplet  Inverting  System. 

greater  eye  distance  than  the  exceedingly  small  eye  lenses  of 
short  focus  oculars,  and  using  a. solid  eyepiece  and  single  lens 
objective  lose  no  more  light  than  an  ordinary  Huygenian  ocular. 
The  erect  resultant  image  is  occasionally  a  convenience  in 
astronomical  use. 

Quite  analogous  to  the  eyepiece   microscope  is  the  so-called 


Fig.  108. — Microscope  as  Ocular. 

"Davon"  micro-telescope.  Originally  developed  as  an  attach- 
ment for  the  substage  of  a  microscope  to  give  large  images 
of  objects  at  a  little  distance  it  has  grown  also  into  a  separate 


Fig.  109. — "Davon"  Instrument. 

hand  telescope,  monocular  or  binocular,  for  general  purposes. 
The  attachment  thus  developed  is  shown  complete  in  Fig.  109. 
D  is  merely  a  well  corrected  objective  set  in  a  mount  provided 
with  ample  stops.  The  image  is  viewed  by  an  ordinary  micro- 
scope or  special  eyepiece  microscope  A,  as  the  case  may  be, 
furnished  with  rack  focussing  at  A'  and  assembled  with  the  objec- 
tive by  means  of  the  carefully  centered  coupling  C. 


EYEPIECES  149 

It  furnishes  a  compact  and  powerful  instrument,  very  suitable 
for  terrestrial  or  minor  astronomical  Uses,  like  the  Tolles'  short- 
focus  hand  telescopes  already  mentioned.  When  properly 
designed  telescopes  of  this  sort  give  nearly  the  field  of  prism 
glasses,  weigh  much  less  and  lose  far  less  light  for  the  same  effec- 
tive power  and  aperture.  They  also  have  under  fairly  high 
powers  rather  the  advantage  in  the  matter  of  definition,  other 
things  being  equal. 


CHAPTER  VII 
HAND  TELESCOPES  AND  BINOCULARS 

The  hand  telescope  finds  comparatively  little  use  in  observing 
celestial  bodies.  It  is  usually  quite  too  small  for  any  except 
very  limited  applications,  and  cannot  be  given  sufficient  power 
without  being  difficult  to  keep  steady  except  by  the  aid  of  a 
fixed  mounting.  Still,  for  certain  work,  especially  the  obser- 
vation of  variable  stars,  it  finds  useful  purpose  if  sufficiently 
compact  and  of  good  light-gathering  power. 

There  is  most  decidedly  a  limit  to  the  magnifying  power  which 
can  be  given  to  an  instrument  held  in  the  hand  without  making 
the  outfit  too  unsteady  to  be  serviceable.  Anything  beyond  8  to 
10  diameters  is  highly  troublesome,  and  requires  a  rudimentary 
mount  or  at  least  steadying  the  hand  against  something  in  order 
to  observe  with  comfort. 

The  longer  the  instrument  the  more  difficult  it  is  to  manage, 
and  the  best  results  with  hand  telescopes  are  to  be  obtained  with 
short  instruments  of  relatively  large  diameter  and  low  power. 
The  ordinary  field  glass  of  Galilean  type  comes  immediately 
to  mind  and  in  fact  the  field  glass  is  and  has  been  much  used.  As 
ordinarily  constructed  it  is  optically  rather  crude  for  astronomical 
purposes.  The  objectives  are  rarely  well  figured  or  accurately 
centered  and  a  bright  star  usually  appears  as  a  wobbly  flare 
rather  than  a  point. 

Furthermore  the  field  is  generally  small,  and  of  quite  uneven 
illumination  from  centre  to  periphery,  so  that  great  caution  has 
to  be  exercised  in  judging  the  brightness  of  a  star,  according  to  its 
position  in  the  field.  The  lens  diameter  possible  with  a  field 
glass  of  ordinary  construction  is  limited  by  the  limited  distance 
between  the  eyes,  which  must  be  well  centered  on  the  eyepieces 
to  obtain  clear  vision. 

The  inter-pupillary  distance  is  generally  a  scant  2^-^  inches  so 
that  the  clear  aperture  of  one  of  the  objectives  of  a  field  glass  is 
rarely  carried  up  to  2  inches.  The  best  field  glasses  have  each 
objective  a  triple  cemented  lens,  and  the  concave  lenses  also 

150 


HAND  TELESCOPES  AND  BINOCULARS 


151 


triplets,  the  arrangement  of  parts  being  that  shown  in  Fig.  110. 
Glasses  of  this  sort  rarely  have  a  magnifying  power  above  5. 

In  selecting  a  field  glass  with  the  idea  of  using  it  on  the  sky 
try  it  on  a  bright  star,  real  or  artificial,  and  if  the  image  with 
careful  focussing  does  not  pull  down  to  a  pretty  small  and  uniform 
point  take  no  further  interest  in  the  instrument. 

The  advantage  of  a  binocular  instrument  is  popularly  much 
exaggerated.  It  gives  a  somewhat  delusive  appearance  of 
brilliancy    and    clearness    which    is    psychological    rather    than 


Fig.  110.— Optical  Parts  of  Field  Glass. 


physical.  During  the  late  war  a  very  careful  research  was  made 
at  the  instance  of  the  United  States  Government  to  determine 
the  actual  value  of  a  binocular  field  glass  against  a  monocular 
one  of  exactly  the  same  type,  the  latter  being  cheaper,  lighter,  and 
in  many  respects  much  handier. 

The  difference  found  in  point  of  actual  seeing  all  sorts  of  objects 
under  varying  conditions  of  illumination  was  so  small  as  to  be 
practically  negligible.  An  increase  of  less  than  5  per  cent  in 
magnifying  power  enabled  one  to  see  with  the  monocular  instru- 
ment everything  that  could  be  seen  with  the  binocular,  equally 
well,  and  it  is  altogether  probable  that  in  the  matter  of  seeing 
fine  detail  the  difference  would  be  even  less  than  in  general  use, 
since  it  is  not  altogether  easy  to  get  the  two  sides  of  a  binocular 
working  together  efficiently  or  to  keep  them  so  afterwards. 

There  has  been,  therefore,  a  definite  field  for  monocular  hand 
telescopes  of  good  quality  and  moderate  power  and  such  are 
manufactured  by  some  of  the  best  Continental  makers.     Such 


152 


THE  TELESCOPE 


instruments  have  sometimes  been  shortened  by  building  them  on 
the  exact  principle  of  the  telephoto  lens,  which  gives  a  relatively 
large  image  with  a  short  camera  extension. 

A  much  shortened  telescope,  as  made  by  Steinheil  for  solar 
photographic  purposes,  is  shown  in  Fig.  111.  This  instrument 
with  a  total  length  of  about  2  feet  and  a  clear  aperture  of  2% 


'--  T--1..  .."I, 


Fig.  111. — Steinheil  Shortened  Telescope. 


inches  gives  a  solar  image  of  3^^  inch  diameter,  corresponding  to  an 
ordinary  glass  of  more  than  double  that  total  length.  Quite  the 
same  principle  has  been  applied  to  terrestrial  telescopes  by  the 
same  maker,  giving  again  an  equivalent  focus  of  about  double  the 
length  of  the  whole  instrument.  This  identical  principle  has 
often  been  used  in  the  so-called  Barlow  lens,  a  negative  lens 
placed  between  objective  and  eyepiece  and  giving  increased 
magnification  with  small  increase  of  length;  also  photographic 
enlargers  of  substantially  similar  function  have  found  consider- 
able use. 

A  highly  efficient  hand  telescope  for  astronomical  purposes 
might  be  constructed  along  this  line,  the  great  shortening  of  the 
instrument  making  it  possible  to  use  somewhat  higher  powers 
than  the  ordinary  without  too  much  loss  of  steadiness.  There  is 
also  constructed  a  binocular  for  strictly  astronomical  use  consist- 
ing of  a  pair  of  small  hand  comet-seekers. 

One  of  these  little  instruments  is  shown  in  Fig.  112.  It  has 
a  clear  diameter  of  objectives  of  1%  inch,  magnification  of  5,  and 
a  brilliant  and  even  field  of  73^^°  aperture.  The  objectives  are 
triplets  like  Fig.  57,  already  referred  to,  the  oculars  achromatic 
doublets  of  the  type  of  Fig.  104a. 

With  the  exception  of  these  specialized  astronomical  field 
glasses  the  most  useful  and  generally  available  hand  instrument 
is  the  prism  glass  now  in  very  general  use.  It  is  based  on  reversal 
of  the  image  by  internal  total  reflection  in  two  prisms  having 
their  reflecting  surfaces  perpendicular  each  to  the  other.     The 


HAND  TELESCOPES  AND  BINOCULARS 


153 


Fig.  112. — Astronomical 
Binocular. 


rudiments  of  the  process  lie  in  the  simple  reversion  prism  shown 
in  diagram  in  Fig.  113. 

This  is  nothing  more  nor  less  than  a  right  angled  glass  prism 
set  with  its  hypothenuse  face  parallel  and  with  its  sides  at  45°  to 
the  optical  axis  of  the  instrument. 
Rays  falling  upon  one  of  its  refracting 
faces  at  an  angle  of  45°  are  refracted 
upon  the  hypothenuse  face,  are  there 
totally  reflected  and  emerge  from  the 
second  face  of  the  prism  parallel  to 
their  original  course. 

Inspection  of  Fig.  113  shows  that 
an  element  like  A  B  perpendicular  to 
the  plane  of  the  hypothenuse  face  is 
inverted  by  the  total  reflection  so  that 
it  takes  the  position  A'  B'.  It  is 
equally  clear  that  an  element  exactly 
perpendicular  to  A  B  will  be  reflected 
from  the  hypothenuse  face  flat-wise 
as  it  were,  and  will  emerge  without  its  ends  being  reversed  so 
that  the  net  effect  of  this  single  reflection  is  to  invert  the  image 
without  reversing  it  laterally  at  the  same  time. 

On  the  other  hand  if  a  second  prism  be  placed  behind  the 
first,  flat  upon  its  side,  with  its  hypothenuse  face  occupying  a 
plane  exactly  perpendicular  to  that  of  the  first  prism,  the  fine 
A'B'  will  be  refracted,  totally  reflected  and  refracted  again  out  of 
the  prism  without  a  second  inversion,  while  a  line  perpendicular 
to  A'B'  will  be  refracted  endwise  on  the  hypothenuse  face  of 

the  second  prism  and  will  be 
inverted  as  was  the  line  A  B 
at  the  start. 

Consequently  two  prisms 
thus  placed  will  completely 
invert  the  image,  producing 
exactly  the  same  effect  as  the 
ordinary  inverting  system  Fig.  106.  The  simple  reversion  prism 
is  useful  as  furnishing  a  means,  when  placed  over  an  eye  lens,  and 
rotated,  of  revolving  the  image  on  itself,  a  procedure  occasionally 
convenient,  especially  in  stellar  photometry.  The  two  prisms 
together  constitute  a  true  inverting  system  and  have  been  utilized 
in  that  function,  but  they  give  a  rather  small  angular   field   and 


Fig.  113. — Reversion  Prism. 


154 


THE  TELESCOPE 


have  never  come  into  a  material  amount  of  use.  The  exact 
effect  of  this  combination,  known  historically  as  Dove's  prisms, 
is  shown  plainly  in  Fig.  114. 

The  first  actual  prismatic  inverting  system  was  due  to  M. 
Porro,  who  invented  it  about  the  middle  of  the  last  century,  and 


Fig.  114. — Dove's  Prisms. 

later  brought  it  out  commercially  under  the  name  of  "Lunette  ^ 
Napoleon  Troisieme,"  as  a  glass  for  military  purposes. 

The  prism  system  of  this  striking  form  of  instrument  is  shown 
in  Fig.  115.  It  was  composed  of  three  right  angle  prisms  A,  B, 
and  C.  A  presented  a  cathetus  face  to  the  objective  and  B  a 
cathetus    face    to    the    ocular.     Obviously    a  vertical  element 


Fig.  115. — Porro's  Prism  System. 


brought  in  along  a  from  the  objective  would  be  reflected  at  the 
hypothenuse  face  h,  to  a  position  at  right  angles  to  the  original 
one,  would  enter  the  hypothenuse  face  of  C  and  thence  after 
two  reflections  at  c  and  d  flatwise  and  without  change  of  direction 
would  emerge,  enter  the  lower  cathetus  face  of  B  and  by  reflec- 
tion at  the  hypothenuse  face  e  of  5  would  be  turned  another 
90°  making  a  complete  reversion  as  regards  up  and  down  at  the 
eye  placed  at  /.  An  element  initially  at  right  angles  to  the  one 
just  considered  would  enter  A,  be  reflected  flatwise,  in  the  faces 
of  C  be  twice  reflected  endwise,  thereby  completely  inverting  it, 


HAND  TELESCOPES  AND  BINOCULARS 


155 


and  would  again  be  reflected  flatwise  from  the  hypothenuse 
face  of  C,  thereby  effecting,  as  the  path  of  the  rays  indicated 
plainly  shows,  a  complete  inversion  of  the  image.  Focussing  was 
very  simply  attained  by  a  screw 
motion  affecting  the  prism  C  and 
the  whole  affair  was  in  a  small 
flat  case,  the  external  appearance 
and  size  of  which  is  indicated  in 
Fig.  116. 

From  ocular  to  objective  the 
length  was  about  an  inch  and  a 
half.  It  was  of  10  power  and  took 
in  a  field  of  45  yards  at  a  distance 
of  1000  yards.  Here  for  the  first 
time  we  find  a  prismatic  inverting 
system  of  strictly  modern  type. 
And  it  is  interesting  to  note  that 
if  one  had  wished  to  make  a 
binocular  ''Lunette  a  Napoleon 
Troisieme"  he  would  inevitably 
have  produced  an  instrument  with 
enhanced  stereoscopic  effect  like 
the  modern  prism  field  glass  by  the  mere  effort  to  dodge  the 
observer's  nose.  Somewhat  earlier  M.  Porro  had  arranged  his 
prisms  in  the  present  conventional  form  of  Fig.  117,  where  two 
right  angle  prisms  have  their  faces  positioned  in  parallel  planes, 


Fig.  116. 


-Lunette  &  Napoleon 
Troisieme. 


-t 


Fig.  117. — Porro's  First  Form  of  Prisms. 


but  turned  around  by  90°  as  in  Fig.  114.  The  ray  traced  through 
this  conventional  system  shows  that  exactly  the  same  inversion 
occurs  here  as  in  the  original  Porro  construction,  and  this  form 
is  the  one  which  has  been  most  commonly  used  for  prismatic 
inversion  and  is  conveniently  known  as  Porro's  first  form,  it 
actually  having  been  antecedent  in  principle  and  practice  to  the 


156  THE  TELESCOPE 

"Lunette  a  Napoleon  Troisieme."  The  original  published 
description  of  Porro's  work,  translated  from  "Cosmos"  Vol.  2, 
■p.  222  (1852)  et  seq.  is  here  annexed  as  it  sets  forth  the  origin 
of  the  modern  prism  glass  in  unmistakable  terms. 

Cos7nos,  Vol.  2,  p.  222. — ''We  have  wished  for  some  time  to  make 
known  to  our  readers  the  precious  advantages  of  the  "longue-vue 
cornet"  or  telemetre  of  M.  Porro.  Ordinary  spyglasses  or  ter- 
restrial telescopes  of  small  dimensions  are  at  least  30  or  40  cm. 
long  when  extended  to  give  distinct  vision  of  distant  objects. 
The  length  is  considerably  reduced  by  substituting  for  a  fixed 
tube  multiple  tubes  sliding  into  each  other.  But  the  drawing 
out  which  this  substitution  necessitates  is  a  somewhat  grave 
inconvenience;  one  cannot  point  the  telescope  without  arranging 
it  and  losing  time. 

For  a  long  time  we  have  wished  it  were  possible  to  have  the 
power  of  viewing  distant  objects,  with  telescopes  very  short  and 
without  draw.  M.  Porro's  "longue-vue  cornet"  seems  to  us  to 
solve  completely  this  difficult  and  important  problem.  Its  con- 
struction rests  upon  an  exceedingly  ingenious  artifice  which  liter- 
ally folds  triply  the  axis  of  the  telescope  and  the  luminous  ray  so 
that  by  this  fact  alone  the  length  of  the  instrument  is  reduced  by 
two-thirds. 

Let  us  try  to  give  an  idea  of  this  construction:  Behind  the 
telescope  objective  M.  Porro  places  a  rectangular  isosceles 
prism  of  which  the  hypothenuse  is  perpendicular  to  the  optic 
axis.  The  luminous  rays  from  the  object  fall  upon  the  rectangu- 
lar faces  of  this  prism,  are  twice  totally  reflected,  and  return  upon 
themselves  parallel  to  their  original  direction:  half  way  to  the 
point  where  they  would  form  the  image  of  the  object,  they  are 
arrested  by  a  second  prism  entirely  similar  to  the  first,  which 
returns  them  to  their  original  direction  and  sends  them  to  the 
eyepiece  through  which  we  observe  the  real  image.  If  the  rec- 
tangular faces  of  the  second  prism  were  parallel  to  the  faces  of  the 
first,  this  real  image  would  be  inverted — the  telescope  would  be 
an  astronomical  and  not  a  terrestrial  telescope.  But  M.  Porro 
being  an  optician  eminently  dextrous,  well  divined  that  to 
effect  the  reinversion  it  sufficed  to  place  the  rectangular  faces  of 
the  second  prism  perpendicular  to  the  corresponding  faces  of  the 
first  by  turning  them  a  quarter  revolution  upon  themselves. 

In  effect,  a  quarter  revolution  of  a  reflecting  surface  is  a  half 
revolution  for  the  image,  and  a  half  revolution  of  the  image 


HAND  TELESCOPES  AND  BINOCULARS  157 

evidently  carries  the  bottom  to  the  top  and  the  right  to  the  left, 
effecting  a  complete  inversion.  As  the  image  is  thus  redressed 
independently  of  the  eyepiece  one  can  evidently  view  it  with  a 
simple  two-lens  ocular  which  decreases  still  further  the  length 
of  the  telescope  so  that  it  is  finally  reduced  to  about  a  quarter 
of  that  of  a  telescope  of  equal  magnifying  power,  field  and 
clearness. 

The  new  telescope  is  then  a  true  pocket  telescope  even  with 
a  magnifying  power  of  10  or  15.  Its  dimensions  in  length  and 
bulk  are  those  of  a  field  glass  usually  magnifying  only  4  to  6 
times.  The  more  draws,  the  more  bother, — it  here  suffices  to 
turn  a  little  thumbscrew  to  find  in  an  instant  the  point  of 
sharpest  vision. 

In  brilliancy  necessarily  cut  down  a  little,  not  by  the  double 
total  reflection,  which  as  is  well  known  does  not  lose  hght,  but 
by  the  quadruple  passage  across  the  substance  of  the  two  prisms, 
the  cornet  in  sharpness  and  amplification  of  the  images  can 
compare  with  the  best  hunting  telescopes  of  the  celebrated 
optician  Ploessl  of  Vienna.  M.  Porro  has  constructed  upon  the 
same  principles  a  marine  telescope  only  15  cm.  long  with  an 
objective  of  40  m.m.  aperture  which  replaces  an  ordinary  marine 
glass  70  cm.  long.  He  has  done  still  better, — a  telescope  only 
30  cm.  long  carries  a  60  m.m.  objective  and  can  be  made  by  turns 
a  day  and  a  night  glass,  by  substituting  by  a  simple  movement  of 
the  hand  and  without  dismounting  anything,  one  ocular  for  the 
other.  Its  brilliancy  and  magnification  of  a  dozen  times  with 
the  night  ocular,  of  twenty-five  times  with  the  day  ocular, 
permits  observing  without  difficulty  the  eclipses  of  the  satellites 
of  Jupiter. 

This  is  evidently  immense  progress.  One  of  the  most  illustri- 
ous of  German  physicists,  M.  Dove  of  Berlin,  gave  in  1851  the 
name  of  reversion  prism  to  the  combination  of  two  prisms  placed 
normally  one  behind  the  other  so  that  their  corresponding  faces 
were  perpendicular.  He  presented  this  disposition  as  an  im- 
portant new  discovery  made  by  himself.  He  doubtless  did  not 
know  that  M.  Porro,  who  deserves  all  the  honor  of  this  charming 
application,  had  realized  it  long  before  him." 

A  little  later  M.  Porro  produced  what  is  commonly  referred  to 
as  Porro's  second  form,  which  is  derived  directly  from  annexing  A 
Fig.  115  to  the  corresponding  half  of  C  as  a  single  prism,  the 
other  half  of  B  being  similarly  annexed  to  the  prism  C,  thus  form- 


158 


THE  TELESCOPE 


ing  two  sphenoid  prisms,  such  as  are  shown  in  Fig.  118 
which  may  be  mounted  separately  or  may  have  their  faces 
cemented  together  to  save  loss  of  light  by 
reflections.  The  sphenoid  prisms  have 
had  the  reputation  of  being  much  more 
difficult  to  construct  than  the  plain  right 
angled  prisms  of  the  other  forms  shown. 
In  point  of  fact  they  are  not  particularly 
difficult  to  make  and  the  best  inverting 
eye  pieces  for  telescopes  are  now  con- 
structed with  sphenoid  prisms  like  those 
just  described. 

■    This  particular  arrangement  lends  itself  very  readily  to  a  fairly 
compact  and  symmetrical  mounting,  as  is  well  shown  in  Fig.  119 


Fig.  118.— Porro's 
Second  Form. 


Fig.  119. — Clark  Prismatic  Eyepiece. 


which  is  the  terrestrial  prismatic  eyepiece  as  constructed  by  the 
Alvan  Clark  corporation  for  application  to  various  astronomical 
telescopes  of  their  manufacture.  A  glance  at  the  cut  shows  the 
compactness  of  the  arrangement,  which  actually  shortens  the 
linear  distance  between  objective  and  ocular  by  the  amount  of 


HAND  TELESCOPES  AND  BINOCULARS 


159 


the  path  of  the  ray  through  the  prisms  instead  of  lengthening  the 
distance  as  in  the  common  terrestrial  eyepiece. 

The  field  moreover  is  much  larger  than  that  attainable  by  a 
construction  like  Fig.  110,  extending  to  something  over  40°, 
and  there  is  no  strong  tendency  for  the  illumination  or  definition 
to  fall  off  near  the  edge  of  the  field. 

In  the  practical  construction  of  prism  field  glasses  the  two.right 
angled  prisms  are  usually  separated  by  a  moderate  space  as  in 
Porro's  original  instruments  so  as  to  shorten  the  actual  length 


Fig.  120. — Section  of  Prism  Binocular. 


of  the  prism  telescope  by  folding  the  ray  upon  itself  as  in  Fig.  120, 
which  is  a  typical  modern  binocular  of  this  class. 

The  path  of  the  rays  is  plainly  shown  and  the  manner  in  which 
the  prisms  fold  up  the  total  focal  length  of  the  objective  is  quite 
obvious.  The  added  stereoscopic  effect  obtained  by  the  arrange- 
ment of  the  two  sides  of  the  instrument  is  practically  a  very 
material  gain.  It  gives  admirable  modelling  of  the  visible  field, 
a  perception  of  distance  which  is  at  least  very  noticeable  and  a 
certain  power  of  penetration,  as  through  a  mass  of  underbrush, 
which  results  from  the  objectives  to  a  certain  extent  seeing  around 
small  objects  so  that  one  or  the  other  of  them  gives  an  image  of 
something  beyond.  For  near  objects  there  is  some  exaggeration 
of  stereoscopic  effect  but  on  the  whole  for  terrestrial  use  the  net 
gain  is  decidedly  in  evidence. 

A  well  made  prism  binocular  is  an  extremely  useful  instrument 
for  observation  of  the  heavens,  provided  the  objectives  are  of  fair 


160  THE  TELESCOPE 

size,  and  the  prisms  big  enough  to  receive  the  whole  beam  from 
the  objective,  and  well  executed  enough  to  give  a  thoroughly 
good  image  with  a  flat  field. 

The  weak  points  of  the  prism  glass  are  great  loss  of  light 
through  reflection  at  the  usual  10  air-glass  surfaces  and  the 
general  presence  of  annoying  ghosts  of  bright  objects  in  the 
field.  Most  such  binoculars  have  Kellner  eyepieces  which  are 
peculiarly  bad,  as  we  have  seen,  with  respect  to  reflected  images, 
and  present  the  plane  surface  of  the  last  prism  to  the  plane 
front  of  the  field  lens.  Recently  some  constructors  have  utilized 
the  orthoscopic  eyepiece,  Figure  105a,  as  a  substitute  with  great 
advantage  in  the  matter  of  reflections. 

The  loss  of  light  in  the  prism  glass  is  really  a  serious  matter, 
between  reflection  at  the  surfaces  and  absorption  in  the  thick 
masses  of  glass  necessary  in  the  prisms.  If  of  any  size  the  trans- 
mitted light  is  not  much  over  one-half  of  that  received,  very 
seldom  above  60%.  If  the  instrument  is  properly  designed  the 
apparent  field  is  in  the  neighborhood  of  45°,  substantially  flat 
and  fairly  evenly  illuminated.  Warning  should  here  be  given 
however  that  many  binoculars  are  on  the  market  in'which  the 
field  is  far  from  flat  and  equally  far  from  being  uniform. 

In  many  instances  the  prisms  are  too  small  to  take  the  whole 
bundle  of  rays  from  the  objective  back  to  the  image  plane  with- 
out cutting  down  the  marginal  light  considerably.  Even  when 
the  field  is  apparently  quite  flat  this  fault  of  uneven  illumina- 
tion may  exist,  and  in  a  glass  for  astronomical  uses  it  is  highly 
objectionable. 

Before  picking  out  a  binocular  for  a  study  of  the  sky  make  very 
careful  trial  of  the  field  with  respect  to  flatness  and  clean  defini- 
tion of  objects  up  to  the  very  edge.  Then  judge  as  accurately 
as  you  may  of  the  uniformity  of  illumination,  if  possible  by  obser- 
vation on  two  stars  about  the  radius  of  the  field  apart.  It  should 
be  possible  to  observe  them  in  any  part  of  the  field  without 
detectable  change  in  their  apparent  brilliancy. 

If  the  objectives  are  easily  removable  unscrew  one  of  them  to 
obtain  a  clear  idea  as  to  the  actual  size  of  the  prisms.^  Look  out, 
too,  for  ghosts  of  bright  stars. 

^  There  are  binoculars  on  the  market  which  are  to  outward  appearance 
prism  glasses,  but  which  are  really  ordinary  opera  glasses  mounted  with  intent 
to  deceive,  sometimes  bearing  a  slight  variation  on  the  name  of  some  well 
known  maker. 


HAND  TELESCOPES  AND  BINOCULARS  161 

The  objectives  of  prism  glasses  usually  run  from  %:  inch  to  1^^^ 
inch  in  diameter,  and  the  powers  from  6  to  12,  The  bigger  the 
objectives  the  better,  provided  the  prisms  are  of  ample  size,  while 
higher  power  than  6  or  8  is  generally  unnecessary  and  disadvan- 
tageous. Occasional  glasses  of  magnifying  power  12  to  20  or 
more  are  to  be  found  but  such  powers  are  inconveniently  great 
for  an  instrument  used  without  support.  Do  not  forget  that  a 
first  class  monocular  prism  glass  is  extremely  xionvenient  and 
satisfactory  in  use,  to  say  nothing  of  being  considerably  less  in 
price  than  the  instrument  for  two  eyes.     A  monocular  prism 


Fig.  121. — Binocular  with  Extreme  Stereoscopic  Effect. 

glass,  by  the  way,  makes  an  admirable  finder  when  fitted  with 
coarse  cross  lines  in  the  eyepiece.  It  is  particularly  well  suited 
to  small  telescopes  without  circles. 

Numerous  modifications  of  Porro's  inverting  prisms  have  been 
made  adapting  them  to  different  specific  purposes.  Of  these  a 
single  familiar  example  will  suffice  as  showing  the  way  in  which 
the  Porro  prism  system  can  be  treated  by  mere  rearrangement  of 
the  prismatic  elements.  In  Fig.  121  is  shown  a  special  Zeiss 
binocular  capable  of  extreme  stereoscopic  effect.  It  is  formed  of 
two  Porro  prism  telescopes  with  the  rays  brought  into  the  objec- 
tives at  right  angles  to  the  axis  of  the  instrument  by  a  right  angled 
prism  external  to  the  objective. 

The  apertures  of  these  prisms  appear  pointing  forward  in  the 
cut.  As  shown  they  are  in  a  position  of  maximum  stereoscopic 
effect. 

Being  hinged  the  tubes  can  be  swung  up  from  the  horizontal 
position,  in  which  latter  the  objectives  are  separated  by  some- 
thing like  eight  times  the  interocular  distance.  The  stereoscopic 
effect  with  the  tubes  horizontal  is  of  course  greatly  exaggerated 
so  that  it  enables  one  to  form  a  fair  judgment  as  to  the  relative 
position  of  somewhat  distant  objects,  a  feature  useful  in  locating 
shell  bursts. 

The  optical  structure  of  one  of  the  pair  of  telescopes  is  shown  in 

Fig.  122  in  which  the  course  of  the  entering  ray  can  be  traced 
11 


162 


THE  TELESCOPE 


through  the  exterior  prism  of  the  objective  and  the  remainder  of 
the    reversing  train    and    thence    through  the  eyepiece.     This 

prism  erecting  system  is  obviously 
derived  from  the  ''Lunette  d 
Napoleon  Troisieme"  by  bringing 
down  the  prism  B  upon  the  corre- 
sponding half  ^  and  cementing  it 
thereto,  meanwhile  placing  the 
objective  immediately  under  A. 
One  occasionally  meets  prismatic 

Fig.  122.— Path  of  Ray  in  Fig.  121.     •  ,-  ,  !.«.     ■ 

inverting  systems  dmermg  con- 
siderably from  the  Porro  forms.  Perhaps  the  best  known  of 
these  is  the  so  called  roof  prism  due  to  Prof.  Abbe,  Fig.  123,  and 
occasionally  useful  in  that  the  entering  and  emerging  rays  lie  in 
the  same  straight  line,  thus  forming  a  direct  vision  system. 
Looking  at  it  as  we  did  at  the  Porro  system  a  vertical  element  in 
front  of  the  prism  is  reversed  in  reflection  from  the  two  surfaces 
a  and  b,  while  a  corresponding  horizontal  element  is  reflected 
flatwise  so  far  as  these  are  concerned,  but  is  turned  end  for  end  by 
reflection  at  the  roof  surfaces  c  and  d,  thus  giving  complete 
inversion. 


Fig.  123. — Abbe  Roof  Prism. 

In  practice  the  prism  is  made  as  shown,  In  three  parts,  two  of 
them  right  angled  prisms,  the  third  containing  the  roof  surfaces. 
The  extreme  precision  required  in  figuring  the  roof  forms  a  con- 
siderable obstacle  to  the  production  of  such  prisms  in  quantity 
and  while  they  have  found  convenient  use  in  certain  special 
instruments  like  gunsights,  where  direct  vision  is  useful,  they  are 
not  extensively  employed  for  general  purposes,  although  both 
monocular  and  binocular  instruments  have  been  constructed 
by  their  aid. 

One  other  variety  of  prism  involving  the  roof  principle  has 
found  some  application  in  field  glasses  manufactured  by  the  firm 


HAND  TELESCOPES  AND  BINOCULARS 


163 


Fig.  124. — Hensoldt  Prism. 


of  Hensoldt.  The  prism  form  used  is  shown  in  Fig.  124.  This 
Hke  other  forms  of  roof  prism  is  less  easy  to  make  than  the  con- 
ventional Porro  type.  Numerous  inverting  and  laterally  reflect- 
ing prisms  are  in  use  for  specific  purposes.  Some  of  them  are  highly 
ingenious  and  remarkably  well  adapted 
for  their  use,  but  hardly  can  be  said  to 
form  a  material  portion  of  telescope 
practice.  They  belong  rather  to  the 
technique  of  special  instruments  like 
gunsights  and  periscopes,  while  some  of 
them  have  been  devised  chiefly  as  in- 
genious substitutes  for  the  simpler  Porro 
forms. 

Most  prism  telescopes  both  mon- 
ocular and  binocular  are  generally  made 
on  one  or  the  other  of  the  Porro  forms. 
This  is  particularly  true  of  the  large 
binoculars  which  are  occasionally  constructed.  Porro's  second 
form  with  the  sphenoid  prisms  seems  to  be  best  adapted  to 
cases  where  shortening  of  the  instrument  is  not  a  paramount 
consideration.  For  example,  some  Zeiss  short  focus  telescopes 
are  regularly  made  in  binocular  form,  and  supplied  with  inverting 
systems  composed  of  two  sphenoid  prisms,  and  with  oculars  con- 
structed on  the  exact  principle  of  the  triple  nose-piece  of  a 
microscope,  so  that  three  powers  are  immediately  available 
to  the  observer. 

Still  less  commonly  binocular  telescopes  of  considerable 
aperture  are  constructed,  primarily  for  astronomical  use,  being 
provided  with  prismatic  inversion  for  terrestrial  employment, 
but  more  particularly  in  order  to  gain  by  the  lateral  displacement 
of  a  Porro  system  the  space  necessary  for  two  objectives  of 
considerable  size.  As  we  have  already  seen,  the  practical 
diameter  of  objectives  in  a  binocular  is  limited  to  a  trifle  over 
2  inches  unless  space  is  so  gained.  The  largest  prismatic  binocular 
as  yet  constructed  is  one  made  years  ago  by  the  Clarks,  of  63-^ 
inches  objective  aperture  and  923^^  inches  focal  length.  So  big  and 
powerful  an  instrument  obviously  would  give  admirable  binocular 
views  of  the  heavens  and  so  accurately  was  it  constructed  that  the 
reports  of  its  performance  were  exceedingly  good.  The  same 
firm  has  made  a  good  many  similar  binoculars  of  3  inch  and 
above,  of  which  a  typical  example  of  4  inch  aperture  and  60 


164 


THE  TELESCOPE 


inch  focal  length  is  shown  in  Fig.  125.     In  this  case  the  erecting 
systems  were  of  Porro's  first  form,  and  were  provided  with  Kellner 


Fig.  125. — Clark  4"  Binocular  Telescope. 


oculars  of  very  wide  field.  These  binoculars  constructions  in 
instruments  of  such  size,  however  well  made  and  agreeable  for 
terrestrial  observation,  hardly  justify  the  expense  for  purely 
astronomical  use. 


CHAPTER    VIII 


ACCESSORIES 


Aside  from  the  ordinary  equipment  of  oculars  various  acces- 
sories form  an  important  part  of  the  observer's  equipment,  their 
number  and  character  depending  on  the  instrument  in  use  and 
the  purposes  to  which  it  is  devoted. 

First  in  general  usefulness  are  several  special  forms  of  eyepiece 
equipment  supplementary  to  the  usual  oculars.     At  the  head  of 


"^S^ 


Fig.  126. — Star  Diagonal. 

the  list  is  the  ordinary  star  diagonal  for  the  easier  viewing  of 
objects  near  the  zenith  here  shown  in  Fig.  126.  It  is  merely  a 
tube,  A,  fitting  the  draw  tube  of  the  telescope,  with  a  slotted  side 
tube  B,  at  a  right  angle,  into  which  the  ordinary  ocular  fits,  and  a 
right  angled  prism  C  with  its  two  faces  perpendicular  respectively 
to  the  axes  of  the  main  and  side  tubes,  and  the  hypothenuse  face 
at  45°  to  each.  The  beam  coming  down  the  tube  is  totally 
reflected  at  this  face  and  brought  to  focus  at  the  ocular.  The 
lower  end  of  the  tube  is  closed  by  a  cap  to  exclude  dust. 

One  looks,  by  help  of  this,  horizontally  at  zenith  stars,  or,  if 
observing  objects  at  rather  high  altitude,  views  them  at  a 
comfortable  angle  downward.     The  prism  must  be  very  accu- 

165 


166 


THE  TELESCOPE 


rately  made  to  avoid  injury  to  the  definition,  but  loses  only 
about  10%  of  the  light,  and  adds  greatly  to  the  comfort  of 
observing. 

Of  almost  equal  importance  is  the  solar  diagonal  devised  by 
Sir  John  Herschel,  Fig.  127.  Here  the  tube  structure  A,  B,  is 
quite  the  same  as  in  Fig.  126  but  the  right  angled  prism  is 
replaced  by  a  simple  elliptical  prism  C  of  small  angle,  10°  or  less, 
with  its  upper  face  accurately  plane  and  at  45°  to  the  axes  of  the 


Fig.  127. — Solar  Diagonal. 


tubes,  resting  on  a  hning  tube  D  cut  off  as  shown.  In  viewing 
the  sun  only  about  5%  of  the  light  (and  heat)  is  reflected  at 
this  upper  surface  to  form  the  image  at  the  eye  piece. 

Any  reflection  from  the  lower  polished  surface  is  turned  aside 
out  of  the  field,  while  the  remainder  of  the  radiation  passes 
through  the  prism  C  and  is  concentrated  below  it.  To  prevent 
scorching  the  observer  the  lower  end  of  the  tube  is  capped  at  E, 
but  the  cap  has  side  perforations  to  provide  circulation  for  the 
heated  air.  Using  such  a  prism,  the  remnant  of  light  reflected 
can  be  readily  toned  down  by  a  neutral  tinted  glass  over  the 
ocular. 

In  the  telescopes  of  3  inches  and  less  aperture,  and  ordinary  focal 
ratio,  a  plane  parallel  disc  of  very  dark  glass  over  the  ocular  gives 
sufficient  protection  to  the  eye.  This  glass  is  preferably  of 
neutral  tint,  and  commonly  is  a  scant  }iQ  inch  thick.  Some 
observers  prefer  other  tints  than  neutral.  A  green  and  a  red  glass 
superimposed  give  good  results  and  so  does  a  disc  of  the  deepest 
shade  of  the  so-called  Noviweld  glass,  which  is  similar  in  effect. 

With  an  aperture  as  large  as  3  inches  a  pair  of  superimposed  dark 


ACCESSORIES 


167 


glasses  is  worth  while,  for  the  two  will  not  break  simultaneously 
from  the  heat  and  there  will  be  time  to  get  the  eye  away  in 
safety.  A  broken  sunshade  is  likely  to  cost  the  observer  a 
permanent  scotoma,  blindness  in  a  small  area  of  the  retina  which 
will  neither  get  better  nor  worse  as  time  goes  on. 

Above  3  inches  aperture  the  solar  prism  should  be  used  or,  if  one 
cares  to  go  to  fully  double  the  cost,  there  is  nothing  more  com- 
fortable to  employ  in  solar  observa- 
tion than  the  polarizing  eye  piece, 
Fig.  128.  This  shows  schematically 
the  arrangement  of  the  device.  It 
depends  on  the  fact  that  a  ray  of  light 
falling  on  a  surface  of  common  glass 
at  an  angle  of  incidence  of  approxi- 
mately 57°  is  polarized  by  the  reflec- 
tion so  that  while  it  is  freely  reflected 
if  it  falls  again  on  a  surface  parallel  to 
the  first,  it  is  absorbed  if  it  falls  at  the 
same  incidence  on  a  surface  at  right 
angles  to  the  first. 

Thus  in  Fig.  128  the  incident  beam 
from  the  telescope  falls  on  the  black 
glass  surface  a  at  57°  incidence,  is 
again  reflected  from  the  parallel  mirror 
h,  and  then  passed  on,  parallel  to  its 
original  path,  to  the  lower  pair  of 
mirrors  c,  d.  The  purpose  of  the  second  reflection  is  to  polarize 
the  residual  light  which  through  the  convergence  of  the  rays 
was  incompletely  polarized  at  the  first. 

The  lower  pair  of  mirrors  c,  d,  again  twice  reflect  the  light  at  the 
polarizing  angle,  and,  in  the  position  shown,  pass  it  on  to  the 
ocular  diminished  only  by  the  four  reflections.  But  if  the  second 
pair  of  mirrors  be  rotated  together  about  a  line  parallel  to  5  c  as  an 
axis  the  transmitted  light  begins  to  fade  out,  and  when  they  have 
been  turned  90°,  so  that  their  planes  are  inclined  90°  to  a  and  b 
(=  33°  to  the  plane  of  the  paper),  the  light  is  substantially 
extinguished. 

Thus  by  merely  turning  the  second  pair  of  mirrors  the  solar 
image  can  be  reduced  in  brilliancy  to  any  extent  whatever, 
without  modifying  its  color  in  any  way.  The  typical  form  given 
to  the  polarizing  eyepiece  is  similar  to  Fig.  129.     Here  ^2  is  the 


Fig.  128. — Diagram  of  Polar- 
izing Eyepiece. 


168 


THE  TELESCOPE 


box  containing  the  polarizing  mirrors,  a  h,  fitted  to  the  draw 
tube,  but  for  obvious  reasons  eccentric  with  it,  ^i  is  the  rotating 
box  containing  the  "analysing"  mirrors  c,  d,  and  a  is  the  ocular 
turning  with  it. 

Sometimes  the  polarizing  mirrors  are  actually  a  pair  of  Herschel 
prisms  as  in  Fig.  126,  facing  each  other,  thus  getting  rid  of  much 


Fig.  129. — Polarizing  Solar  Eyepiece. 


of  the  heat.  Otherwise  the  whole  set  of  mirrors  is  of  black  glass 
to  avoid  back  reflections.  In  simpler  constructions  single  mirrors 
are  used  as  polarizer  and  analyser,  and  in  fact  there  are  many 
variations  on  the  polarizing  solar  eyepiece  involving  about  the 
same  principles. 

In  any  solar  eyepiece  a  set  of  small  diaphragms  with  holes 
from  perhaps  3'^4  inch  up  are  useful  in  cutting  down  the  general 
glare  from  the  surface  outside  of  that  under  scrutiny.  These 
may  be  dropped  upon  the  regular  diaphragm  of  the  ocular  or 
conveniently  arranged  in  a  revolving  diaphragm  like  that  used 
with  the  older  photographic  lenses. 

The  measurement  of  celestial  objects  has  developed  a  large 
group  of  important  auxiliaries  in  the  micrometers  of  very  varied 
forms.  The  simplest  needs  little  description,  since  it  consists 
merely  of  a  plane  parallel  disc  of  glass  fitting  in  the  focus  of  a 
positive  ocular,  and  etched  with  a  network  of  uniform  squares, 


ACCESSORIES  169 

forming  a  reticulated  micrometer  by  which  the  distance  of 
one  object  from  another  can  be  estimated. 

It  can  be  readily  calibrated  by  measuring  a  known  distance 
or  noting  the  time  required  for  an  equatorial  star  to  drift  across 
the  squares  parallel  to  one  set  of  lines.  It  gives  merely  a  useful 
approximation,  and  accurate  measures  must  be  turned  over  to 
more  precise  instruments. 

The  ring  micrometer  due,  like  so  much  other  valuable  apparatus, 
to  Fraunhofer,  is  convenient  and  widely  used  for  determining 
positions.     It  consists,  as  shown  in  Fig.  130,  of  an  accurately 


Fig.  130. — Diagram  of  Ring  Micrometer. 

turned  opaque  ring,  generally  of  thin  steel,  cemented  to  a  plane 
parallel  glass  or  otherwise  suspended  in  the  center  of  the  eyepiece 
field.  The  whole  ring  is  generally  half  to  two  thirds  the  width  of 
the  field  and  has  a  moderate  radial  width  so  that  both  the  ingress 
and  the  egress  of  a  star  can  be  conveniently  timed. 

It  depends  wholly  on  the  measurement  of  time  as  the  stars 
to  be  compared  drift  across  the  ring  while  the  telescope  is  fixed, 
and  while  a  clock  or  chronometer  operating  a  sounder  is  a 
desirable  adjunct  one  can  do  pretty  well  with  a  couple  of  stop 
watches  since  only  differential  times  are  required. 

For  full  directions  as  to  its  use  consult  Loomis'  Practical 
Astronomy,  a  book  which  should  be  in  the  library  of  every  one 
who  has  the  least  interest  in  celestial  observations.     Suffice  it  to 


170 


THE  TELESCOPE 


say  here  that  the  ring  micrometer  is  very  simple  in  use,  and  the 
computation  of  the  results  is  quite  easy.  In  Fig.  130  F  is  the 
edge  of  the  field,  R  the  ring,  and  a  b,  a'h',  the  paths  of  the  stars 
s  and  s',  the  former  well  into  the  field,  the  latter  just  within  the 
ring.  The  necessary  data  comprise  the  time  taken  by  each  star 
to  transverse  the  ring,  and  the  radius  of  the  ring  in  angular 
measure,  whence  the  difference  in  R.  A.  or  Dec,  can  be  obtained.^ 
Difference  of  R.  A.  =  }i  (f  -  t)  -  >^  (T'  -  T)  where  (T'  - 
T)  is  the  time  taken  for  transit  of  second  star.  To  obtain 
differences  of  declination  one  declination  should  be  known  at 


Chamber's  "Astronomy"  {Clarendon  Press). 
Fig.  131. — Double  Image  Micrometer.      {Courtesy  of  The  Clarendon  Press.) 

least  approximately,  and  the  second  estimated  from  its  relative 
position  in  the  ring  or  otherwise.  Then  with  these  tentative 
values  proceed  as  follows. 

Put  X  =  angle  a  oh  and  x'  =  angle  a'o'h' 
Also    let    d  =  approximate  declination  of  s  and 
d'  =  approximate  declination  of  s' 

Then  sin  x    =  ^  cos  d  (T'  -  T) 

15 
sin   x'  =  ^  cos  d'  (f  —  t)  and  finally 
^r 

15 
^  r  the  radius  of  the  ring,  is  given  by,  r  =  -^  (f  —  t')  cos 

Dec,  t'  —  t  being  the  seconds  taken  ifor  transit. 


ACCESSORIES  171 

Difference  of  Dec.  =  r  (cos  x'  —  cos  x),  when  both  arcs  are  on 
the  same  side  of  center  of  ring.  If  on  opposite  sides,  Diff.  =  r 
(cos  x'  +  cos  x). 

There  is  also  now  and  then  used  a  square  bar  micrometer, 
consisting  of  an  opaque  square  set  with  a  diagonal  in  the  line  of 
diurnal  motion.  It  is  used  in  much  the  same  way  as  the  ring, 
and  the  reductions  are  substantially  the  same.  It  has  some 
points  of  convenience  but  is  little  used,  probably  on  account  of 
the  great  difficulty  of  accurate  construction  and  the  requirement, 
for  advantageous  use,  that  the  telescope  should  be  on  a  well 
adjusted  equatorial  stand. ^  The  ring  micrometer  works  reason- 
ably well  on  any  kind  of  steady  mount,  requires  no  illumination 
of  the  field  and  is  in  permanent  working  adjustment. 

Still  another  type  of  micrometer  capable  of  use  without  a 
clock-drive  is  the  double  image  instrument.  In  its  usual  form 
it  is  based  on  the  principle  that  if  a  lens  is  cut  in  two  along  a 
diameter  and  the  halves  are  slightly  displaced  along  the  cut  all 
objects  will  be  seen  double,  each  half  of  the  lens  forming  its 
own  set  of  images. 

Conversely,  if  one  choses  two  objects  in  the  united  field  these 
can  be  brought  together  by  sliding  the  halves  of  the  lens  as  before, 
and  the  extent  of  the  movement  needed  measures  the  distance 
between  them.  Any  lens  in  the  optical  system  can  be  thus  used, 
from  the  objective  to  the  eyepiece.  Fig.  131  shows  a  very 
simple  double  image  micrometer  devised  by  Browning  many 
years  ago.  Here  the  lens  divided  is  a  so-called  Barlow  lens,  a 
weak  achromatic  negative  lens  sometimes  used  like  a  telephoto 
lens  to  lengthen  the  focus  and  hence  vary  the  power  of  a  telescope. 

This  lens  is  shown  at  A  with  the  halves  widely  separated  by  the 
double  threaded  micrometer  screw  B,  which  carries  them  apart 
symmetrically.     The  ocular  proper  is  shown  at  C. 

Double  image  micrometers  are  now  mainly  of  historical 
interest,  and  the  principle  survives  chiefly  in  the  heliometer,  a 
telescope  with  the  objective  divided,  and  provided  with  sliding 
mechanism  of  the  highest  refinement.  The  special  function  of 
the  heliometer  is  the  direct  micrometric  measurement  of  stellar 
distances  too  great  to  be  within  the  practicable  range  of  a  filar 
micrometer — distances  for  example  up  to  13^^°  or  even  more. 

The  observations  with  the  heliometer  are  somewhat  laborious 

^  (For  full  discussion  of  this  instrument  see  Chandler,  Mem.  Amer.  Acad. 
Arts  &  Sci.  1885,  p.  158). 


172 


THE  TELESCOPE 


and  demand  rather  intricate  corrections,  but  are  capable  of  great 
precision.  (See  Sir  David  Gill's  article  "Heliometer"  in  the 
Enc.  Brit.  11th  Ed.).  At  the  present  day  celestial  photography, 
with  micrometric  measurement  of  the  resulting  plates,  has  gone 
far  in  rendering  needless  visual  measurements  of  distances 
above  a  very  few  minutes  of  arc,  so  that  it  is  somewhat  doubtful 
whether  a  large  heliometer  would  again  be  constructed. 

The  astronomer's  real  arm  of  precision  is  the  filar  micrometer. 
This  is  shown  in  outline  in  Fig.  132,  the  ocular  and  the  plate  that 


<r 2: 


Fig.  132. — Filar  Micrometer.     {Courtesy  of  J.  B.  Lippincoit  Co.) 

carries  it  being  removed  so  as  to  display  the  working  parts.  It 
consists  of  a  main  frame  aa,  carrying  a  slide  bb,  which  is  moved 
by  the  screws  and  milled  head  B.  The  slide  bb  carries  the  vertical 
spider  line  mm,  and  usually  one  or  more  horizontal  spider  lines, 
line  mm  is  the  so-called  fixed  thread  of  the  micrometer,  mov- 
able only  as  a  convenience  to  avoid  shifting  the  telescope. 

On  bb  moves  the  micrometer  slide  cc,  carrying  the  movable 
spider  line  nn  and  the  comb  which  records,  with  mm  as  reference 
line,  the  whole  revolutions  of  the  micrometer  screw  C.  The 
ocular  sometimes  has  a  sliding  motion  of  its  own  on  cc,  to  get  it 
positioned  to  the  best  advantage.  In  use  one  star  is  set  upon 
mm  by  the  screw  B  and  then  C  is  turned  until  nn  bisects  the 
other  star. 

Then  the  exact  turns  and  fraction  of  a  turn  can  be  read  off 
on  the  comb  and  divided  head  of  C,  and  reduced  to  angular 
measure  by  the  known  constant  of  the  micrometer,  usually 
determined  by  the  time  of  passage  of  a  nearly  equatorial  star 
along  the  horizontal  thread  when  mm,  nn,  are  at  a  definite  setting 

apart.     (Then  r  =   s;t ' —  where  r  is  the  value  of  a 

revolution  in  seconds  of  arc,  N  the  revolutions  apart  of  mm,  nn, 
and  t  and  d  as  heretofore.) 


ACCESSORIES 


173 


Very  generally  the  whole  system  of  slides  is  fitted  to  a 
graduated  circle,  to  which  the  fixed  horizontal  thread  is  diametral. 
Then  by  turning  the  micrometer  until  the  horizontal  threads  cut 
the  two  objects  under  comparison,  their  position  angle  with 
reference  to  a  graduted  circle  can  be  read  off.  This  angle  is 
conventionally  counted  from  0°  to  360°  from  north  around 
through  east. 

Figure  133  shows  the  micrometer  constructed  by  the  Clarks  for 
their  24  inch  equatorial  of  the  Lowell  Observatory.  Here  A  is  the 
head  of  the  main  micrometer  screw  of  which  the  whole  turns  are 


Fig.  133. — Filar  Position  Micrometer. 


reckoned  on  the  counter  H  in  lieu  of  the  comb  of  Fig.  132.  B  is 
the  traversing  screw  for  the  fixed  wire  system,  C  the  clamping 
screw  of  the  position  circle,  D  its  setting  pinion,  E  the  rack 
motion  for  shifting  the  ocular,  F  the  reading  glass  for  the  position 
circle,  and  G  the  little  electric  lamp  for  bright  wire  illumination. 
The  parts  correspond  quite  exactly  with  the  diagram  of  Fig.  132 
but  the  instrument  is  far  more  elegant  in  design  than  the  earlier 
forms  of  micrometer  and  fortunately  rid  of  the  oil  lamps  that  were 
once  in  general  use.  A  small  electric  lamp  with  reflector  throws 
a  little  light  on  the  spider  lines — just  enough  to  show  them  dis- 
tinctly. Or  sometimes  a  faint  light  is  thus  diffused  in  the 
field  against  which  the  spider  lines  show  dark. 

Commonly  either  type  of  illumination  can  be  used  and  modi- 
fied as  occasion  requires.     The  filar  micrometer  is  seldom  used 


174  THE  TELESCOPE 

on  small  telescopes,  since  to  work  easily  with  it  the  instrument 
should  be  permanently  mounted  and  clock-driven.  Good  work 
was  done  by  some  of  the  early  observers  without  these  aids,  but 
at  the  cost  of  infinite  pains  and  much  loss  of  time. 

The  clock  drive  is  in  fact  a  most  important  adjunct  of  the 
telescope  when  used  for  other  purposes  than  ordinary  visual 
observations,  though  for  simple  seeing  a  smooth  working  slow 
motion  in  R.  A.  answers  well.  The  driving  clock  from  the  horo- 
logical  view-point  is  rudimentary.  It  consists  essentially  of  a 
weight-driven,  or  sometimes  spring-driven,  drum,  turning  by  a 
simple  gear  connection  a  worm  which  engages  a  carefully  cut 
gear  wheel  on  the  polar  axis,  while  prevented  from  running 
away  by  gearing  up  to  a  fast  running  fly-ball  governor,  which 
applies  friction  to  hold  the  clockwork  down  to  its  rate  if  the  speed 
rises  by  a  minute  amount.  There  is  no  pendulum  in  the  ordinary 
sense,  the  regularity  depending  on  the  uniformity  of  the  total 
friction — that  due  to  the  drive  plus  that  applied  by  the  governor. 

Figure  134  shows  a  simple  and  entirely  typical  driving  clock 
by  Warner  &  Swasey.  Here  A  is  the  main  drum  with  its  wind- 
ing gear  at  B,  C  is  the  bevel  gear,  which  is  driven  from  another 
carried  by  A,  and  serves  to  turn  the  worm  shaft  D;  E  marks 
the  fly  balls  driven  by  the  multiplying  gearing  plainly  visible. 
The  governor  acts  at  a  predetermined  rotation  speed  to  lift  the 
spinning  friction  disc  F  against  its  fixed  mate,  which  can  be 
adjusted  by  the  screw  G. 

The  fly-balls  can  be  slightly  shifted  in  effective  position  to 
complete  the  regulation.  These  simple  clocks,  of  which  there  are 
many  species  differing  mainly  in  the  details  of  the  friction  device, 
are  capable  of  excellent  precision  if  the  work  of  driving  the  tele- 
scope is  kept  light. 

For  large  and  heavy  instruments,  particularly  if  used  for  photo- 
graphic work  where  great  precision  is  required,  it  is  difficult  to 
keep  the  variations  of  the  driving  friction  within  the  range  of 
compensation  furnished  by  the  governor  friction  alone,  and  in 
such  case  recourse  is  often  taken  to  constructions  in  which  the 
fly  balls  act  as  relay  to  an  electrically  controlled  brake,  or  in 
which  the  driving  power  is  supplied  by  an  electric  motor  suitably 
governed  either  continuously  or  periodically.  For  such  work 
independent  hand  guiding  mechanism  is  provided  to  supplement 
the  clockwork.  For  equatorials  of  the  smallest  sizes,  say  3  to  4 
inches  aperture,  spring  operated  driving  clocks  are  occasionally 


ACCESSORIES 


175 


used.  The  general  plan  of  operation  is  quite  similar  to  the 
common  weight  driven  forms,  and  where  the  weights  to  be 
carried  are  not  excessive  such  clocks  do  good  work  and  serve  a 
very  useful  purpose. 

An  excellent  type  of  the  simple  spring  driving  clock  is  shown  in 
Fig.  136  as  constructed  by  Zeiss.     Here  1  is  the  winding  gear, 


Fig.  134. — Typical  Driving  Clock.     {Courtesy  of  The  Clarendon  Press.) 


2  the  friction  governor,  and  3  the  regulating  gear.  It  will  be  seen 
that  the  friction  studs  are  carried  by  the  fly  balls  themselves, 
somewhat  as  in  Fraunhofers'  construction  a  century  since,  and 
the  regulation  is  very  easily  and  quickly  made  by  adjusting  the 
height  of  the  conical  friction  surface  above  the  balls. 

For  heavier  work  the  same  makers  generally  use  a  powerful 
weight  driven  train  with  four  fly-balls  and  electric  seconds  control, 
sometimes  with  the  addition  of  electric  motor  slow  motions  to 
adjust  for  R.  A.  in  both  directions. 


176 


THE  TELESCOPE 


Figure  135  is  a  rather  powerful  clock  of  analogous  form  by 
the  Clarks.  It  differs  a  little  in  its  mechanism  and  especially 
in  the  friction  gear  in  which  the  bearing  disc  is  picked  up  by 
a  delicately  set  latch  and  carried  just  long  enough  to  effect 
the  regulation.  It  is  really  remarkable  that  clockworks  of  so 
simple  character  as  these  should  perform  as  well  as  experience 


Fig.  135.— Clark  Driving  Clock. 

shows  that  they  do.  In  a  few  instances  clocks  have  depended 
on  air-fans  for  their  regulating  force,  something  after  the  manner 
of  the  driving  gear  of  a  phonograph,  but  though  rather  success- 
ful for  light  work  they  have  found  little  favor  in  the  task  of 
driving  equatorials.     An  excellent  type  of  a  second  genus  is  the 


ACCESSORIES 


177 


pendulum  controlled  driving  clock  due  to  Sir  David  Gill.  This 
has  a  powerful  weight-driven  train  with  the  usual  fly-ball  gover- 
nor. But  the  friction  gear  is  controlled  by  a  contact-making 
seconds  pendulum  in  the  manner  shown  diagrammatically  in 
Fig.  137.  Two  light  leather  tipped  rods  each  controlled  by  an 
electro  magnet  act  upon  an  auxiliary  brake  disc  carried  by  the 
governor  spindle  which  is  set  for  normal  speed  with  one  brake 
rod  bearing  lightly  on  it.  Exciting  the  corresponding  magnet 
relieves  the  pressure  and  accelerates  the  clock,  while  exciting  the 
other  adds  braking  effect  and  slows  it. 


Fig.  136. — Spring  Operated  Driving  Clock. 

In  Fig.  137  is  shown  from  the  original  paper,  (M.  N.  Nov., 
1873),  the  very  ingenious  selective  control  mechanism.  At  P 
is  suspended  the  contact-making  seconds-pendulum  making 
momentary  contact  by  the  pin  Q  with  a  mercury  globule  at  R. 
Upon  a  spindle  of  the  clock  which  turns  once  a  second  is  fixed  a 
vulcanite  disc  y,  8,  e,  a.  This  has  a  rim  of  silver  broken  at  the 
points  7,  8,  e,  a,  by  ivory  spacers  covering  3°  of  circumference. 
On  each  side  of  this  disc  is  another,  smaller,  and  with  a  complete 
silver  rim.  One,  r]d,  is  shown,  connected  with  the  contact 
spring  V;  its  mate  rj'O',  on  the  other  side  contacts  with  U,  while 
a  third  contact  K  bears  on  the  larger  disc. 

The  pair  of  segments  a,  j,  and  8,  e,  are  connected  to  r?  d,  the 
other  pair  of  segments  to  ri'd'.  Now  suppose  the  discs  turning 
with  the  arrows:  If  K  rests  on  one  of  the  insulated  points  when 
the  pendulum  throws  the  battery  C  Z  into  circuit  nothing  happens. 
If  the  disc  is  gaining  on  the  pendulum,  K,  instead  of  resting  on 

12 


178 


THE  TELESCOPE 


7  as  shown  will  contact  with  segment  7,  a,  and  actuate  a  relay 
via  V,  exciting  the  appropriate  brake  magnet. 

If  the  disc  is  losing,  K  contacts  with  segment  7,  b,  and  current 
will    pass   via   r}'d'   and  U  to  a  relay  that  operates  the  other 


Fig.  137.— Sir  David  Gill's  Electric  Control. 

brake  magnet  and  lets  the  clock  accelerate.  A  fourth  disc 
(not  shown)  on  the  same  spindle  is  entirely  insulated  on  its 
edge  except  at  points  corresponding  to  7  and  e,  and  with  a 
contact  spring  like  K. 

If  the  disc  is  neither  gaining  nor  losing  when  the  pendulum 
makes  contact,  current  flows  via  this  fourth  disc  and  sets  the 


ACCESSORIES  179 

relay  on  the  mid-point  ready  to  act  when  needed.  This  clock 
is  the  prototype  of  divers  electrically-braked  driving  clocks  with 
pendulum  control,  and  proved  beautifully  precise  in  action,  like 
various  kindred  devices  constructed  since,  though  the  whole 
genus  is  somewhat  expensive  and  intricate. 

The  modern  tendency  in  driving  apparatus  for  telescopes, 
particularly  large  instruments,  is  to  utilize  an  electric  motor  for 
the  source  of  power,  using  a  clock  mechanism  merely  for  the 
purpose  of  accurately  regulating  the  rate  of  the  motor.  We  thus 
have  the  driving  clock  in  its  simplest  form  as  a  purely  mechanical 
device  worked  by  a  sensitive  fly-ball  governor.  The  next 
important  type  is  that  in  which  the  clock  drive  is  precisely 
regulated  by  a  pendulum  clock,  the  necessary  governing 
power  being  applied  electrically  as  in  Fig.  137  or  sometimes 
mechanically. 

Finally  we  come  to  the  type  now  under  consideration  where 
the  instrument  itself  is  motor  driven  and  the  function  of  the 
clock  is  that  of  regulating  the  motor.  A  very  good  example  of 
such  a  drive  is  the  Gerrish  apparatus  used  for  practically  all  the 
instruments  at  the  various  Harvard  observatory  stations,  and 
which  has  proved  extremely  successful  even  for  the  most  trying 
work  of  celestial  photography.  The  schematic  arrangement  of 
the  apparatus  is  shown  in  Fig.  138.  Here  an  electric  motor 
shown  in  diagram  in  1,  Fig.  138,  is  geared  down  to  approximately 
the  proper  speed  for  turning  the  right  ascension  axis  of  the 
telescope.  It  is  supplied  with  current  either  from  a  battery  or 
in  practice  from  the  electric  supply  which  may  be  at  hand. 
This  motor  is  operated  on  a  110  volt  circuit  which  supplies 
current  through  the  switch  2  which  is  controlled  by  the  low 
voltage  clock  circuit  running  through  the  magnet  3.  The 
clock  circuit  can  be  closed  and  opened  at  two  points,  one  con- 
trolled by  the  seconds  pendulum  5,  the  other  at  7  by  the  stud 
on  the  timing  wheel  geared  to  the  motor  for  one  revolution 
per  second.  There  is  also  a  shunt  around  the  pendulum  break, 
closed  by  the  magnet  switch  at  6.  This  switch  is  mechanically 
connected  to  the  switch  2  by  the  rod  4,  so  that  the  pair  open  and 
close  together. 

The  control  operates  as  follows:  Starting  with  the  motor  at 
rest,  the  clock  circuit  is  switched  on,  switches  2,  6  being  open 
and  7  closed.  At  the  first  beat  of  the  pendulum  2,  6  closes  and 
the   current,    shunted   across  the  loop    containing   5,   holds  2 


180 


THE  TELESCOPE 


closed  until  the  motor  has  started  and  broken  the  clock  circuit 
at  the  timer.  The  fly-wheel  carries  on  until  the  pendulum 
again  closes  the  power  circuit  via  2,  6,  and  current  stays  on  the 
motor  until  the  timer  has  completed  its  revolution. 

This  goes  on  as  the  motor  speeds  up,  the  periodic  power  supply 
being  shortened  as  the  timer  breaks  it  earlier  owing  to  the  accel- 
eration, until  the  motor  comes  to  its  steady  speed  at  which  the 
power  is  applied  just  long  enough  to  maintain  uniformity.  If 
the  motor  for  any  cause  tends  to  overspeed  the  cut-off  is  earlier, 
while  slowing -d©M*e  produces  a  longer  power-period  bringing  the 


Fig.  138. — Diagram  of  Gerrish  Electric  Control. 

speed  back  to  normal.  The  power  period  is  generally  3^^  to  3^^ 
second.  The  power  supplied  to  the  motor  is  very  small  even  in 
the  example  here  shown,  only  1  ampere  at  110  volts. 

The  actual  proportion  of  a  revolution  during  which  current  is 
supplied  the  motor  is  therefore  rigorously  determined  by  the 
clock  pendulum,  and  the  motor  is  selected  so  that  its  revolutions 
are  exactly  timed  to  this  clock  pendulum  which  has  no  work  to 
do  other  than  the  circuit  closing,  and  can  hence  be  regulated  to 
keep  accurate  time.  The  small  fly-wheel  (9),  the  weight  of  which 
is  carefully  adjusted  with  respect  to  the  general  amount  of  work 
to  be  done,  attached  to  the  motor  shaft,  effectively  steadies  its 
action  during  the  process  of  government.  This  Gerrish  type 
has  been  variously  modified  in  detail  to  suit  the  instruments  to 
which  it  has  been  applied,  always  following  however  the  same 
fundamental  principles. 


ACCESSORIES 


181 


An  admirable  example  of  the  application  of  this  drive  is 
shown  in  Fig.  139,  the  24  inch  reflector  at  the  Harvard  Obser- 
vatory.    The  mount  is  a  massive  open  fork,  and  the  motor  drive 


Fig.  139. — Gerrish  Drive  on  24  inch  Reflector. 


is  seen  on  the  right  of  the  mount.  There  are  here  two  motors, 
ordinary  fan  motors  in  size.  The  right  hand  motor  carries  the 
fly-wheel  and  runs  steadily  on  under  the  pendulum  control.  The 
other,  connected  to  the  same  differential  gear    as  the  driving 


182  THE  TELESCOPE 

motor,  serves  merely  for  independent  regulation  and  can  be  run 
in  either  direction  by  the  observer  to  speed  or  slow  the  motion  in 
R.  A.  These  examples  of  clock  drive  are  merely  typical  of  those 
which  have  proved  to  be  successful  in  use  for  various  service,  light 
and  heavy.  There  are  almost  innumerable  variations  on  clocks 
constructed  on  one  or  another  of  the  general  lines  here  indicated, 
so  many  variations  in  fact  that  one  almost  might  say  there  are 
few  driving  clocks  which  are  not  in  some  degree  special. 

The  tendency  at  present  is  for  large  instruments  very  dis- 
tinctly toward  a  motor-driven  mechanism  operating  on  the  right 
ascension  axis,  and  governed  in  one  of  a  considerable  variety  of 
ways  by  an  actual  clock  pendulum.  For  smaller  instruments  the 
old  mechanical  clock,  often  fitted  with  electric  brake  gear  and 
now  and  then  pendulum  regulated,  is  capable  of  very  excellent 
work. 

The  principle  of  the  spectroscope  is  rudimentarily  simple,  in 
the  familiar  decomposition  of  white  light  into  rainbow  colors  by 
a  prism.  One  gets  the  phenomena  neatly  by  holding  a  narrow 
slit  in  a  large  piece  of  cardboard  at  arms  length  and  looking  at  it 
through  a  prism  held  with  its  edge  parallel  to  the  slit.  If  the 
light  were  not  white  but  of  a  mixture  of  definite  colors  each  color 
present  would  be  represented  by  a  separate  image  of  the  slit 
instead  of  the  images  being  merged  into  a  continuous  colored 
band. 

With  the  sun  as  source  the  continuous  spectrum  is  crossed  by 
the  dark  lines  first  mapped  by  Fraunhofer,  each  representing 
the  absorption  by  a  relatively  cool  exterior  layer  of  some  sub- 
stance that  at  a  higher  temperature  below  gives  a  bright  line  in 
exactly  the  same  position. 

The  actual  construction  of  the  astronomical  spectroscope 
varies  greatly  according  to  its  use.  In  observations  on  the  sun 
the  distant  slit  is  brought  nearer  for  convenience  by  placing  it  in 
the  focus  of  a  small  objective  pointed  toward  the  prisms  (the 
collimator)  and  the  spectrum  is  viewed  by  a  telescope  of  moderate 
magnifying  power  to  disclose  more  of  detail.  Also,  since  there  is 
extremely  bright  light  available,  very  great  dispersion  can  be  used, 
obtained  by  several  or  many  prisms,  so  that  the  spectrum  is 
both  fairly  wide,  (the  length  of  the  slit)  and  extremely  long. 

In  trying  to  get  the  spectrum  of  a  star  the  source  is  a  point, 
equivalent  to  an  extremely  minute  length  of  a  very  narrow  slit. 
Therefore  no  actual  slit  is  necessary  and  the  chief  trouble  is  to 


ACCESSORIES 


183 


Fig.  140. — McClean  Ocular 
Spectroscope. 


get  the  spectrum  wide  enough  and  bright  enough  to  examine. 

The  simplest  form  of  stellar  spectroscope  and  the  one  in  most 
common  use  with  small  telescopes  is  the  ocular  spectroscope 
arranged  much  like  Fig.  140.  This  fits  into  the  eye  tube  of  a 
telescope  and  the  McClean  form  made  by  Browning  of  London 
consists  of  an  ordinary  casing  with  screw  collar  B,  a  cyhndrical 
lens  C,  a  direct  vision  prism  c,  f,  c,  and  an  eye-cap  A. 

The  draw  tube  is  focussed  on  the  star  image  as  with  any  other 
ocular,  and  the  light  is  de- 
livered through  C  to  the  prism 
face  nearly  parallel,  and 
thence  goes  to  the  eye,  after 
dispersion  by  the  prism.  This 
consists  of  a  central  prism,  /, 
of  large  angle,  made  of  ex- 
tremely dense  flint,  to  which 
are  cemented  a  pair  of  prisms 
of  light  crown  c,  c,  with  their 
bases  turned  away  from  that 
of/. 

We  have  already  seen  that  the  dispersions  of  glasses  vary  very 
much  more  than  their  refractions  so  that  with  proper  choice  of 
materials  and  angles  the  refraction  of  /  is  entirely  compensated 
for  some  chosen  part  of  the  spectrum,  while  its  dispersion  quite 
overpowers  that  of  the  crown  prisms  and  gives  a  fairly  long 
available  spectrum. 

The  cylindrical  lens  C  merely  serves  to  stretch  out  the  tiny 
round  star  image  into  a  short  line  thereby  giving  the  resulting 
spectrum  width  enough  to  examine  comfortably.  The  weak 
cylindrical  lens  is  sometimes  slipped  over  the  eye  end  of  the 
prisms  to  give  the  needed  width  of  spectrum  instead  of  putting  it 
ahead  of  the  prisms. 

A  small  instrument  of  this  kind  used  with  a  telescope  of  3 
inches  to  5  inches  aperture  gives  a  fairly  good  view  of  the  spectra 
of  starts  above  second  or  third  magnitude,  the  qualities  of  toler- 
ably bright  comets  and  nebulae  and  so  forth.  The  visibility  of 
spellar  spectra  varies  greatly  according  to  their  type,  those  with 
heavy  broad  bands  being  easy  to  observe,  while  for  the  same 
stellar  magnitude  spectra  with  many  fine  lines  may  be  quite 
beyond  examination.  Nevertheless  a  little  ocular  spectroscope 
enables  one  to  see  many  things  well  worth  the  trouble  of  observing. 


184 


THE  TELESCOPE 


With  the  larger  instruments,  say  6  or  8  inches,  one  can  well  take 
advantage  of  the  greater  light  to  use  a  spectroscope  with  a  slit, 
which  gives  somewhat  sharper  definition  and  also  an  opportunity 
to  measure  the  spectrum  produced. 

An  excellent  type  of  such  an  instrument  is  that  shown  in  Fig. 
141,  due  to  Professor  Abbe.  The  construction  is  analogous  to 
Fig.  140.  The  ocular  is  a  Huyghenian  one  with  the  slit  mechan- 
ism (controlled  by  a  milled  head)  at  A  in  the  usual  place  of  the 

diaphragm.  The  slit  is  therefore 
in  the  focus  of  the  eye  lens, 
which  serves  as  collimating  lens. 
Above  is  the  direct  vision  system 
J  with  the  usual  prisms  which 
are  slightly  adjustable  laterally 
by  the  screw  P  and  spring  Q. 

At  N  is  a  tiny  transparent 
scale  of  wave  lengths  illuminated 
by  a  faint  light  reflected  from 
the  mirror  0,  and  in  the  focus  of 
the  little  lens  R,  which  transfers 
it  by  reflection  from  the  front 
face  of  the  prism  to  the  eye, 
alongside  the  edge  of  the  spec- 
trum. One  therefore  sees  the 
spectrum  marked  off  by  a  bright 
line  wave-length  scale. 
The  pivot  K  and  clamp  L  enable  the  whole  to  be  swung  side- 
wise  so  that  one  can  look  through  the  widened  slit,  locate  the 
star,  close  the  slit  accurately  upon  it  and  swing  on  the  prisms. 
M  is  the  clamp  in  position  angle.  Sometimes  a  comparison 
prism  is  added,  together  with  suitable  means  for  throwing  in 
spectra  of  gases  or  metals  alongside  that  of  the  star,  though  these 
refinements  are  more  generally  reserved  for  instruments  of 
higher  dispersion. 

To  win  the  advantage  of  accurate  centering  of  the  star  in  the 
field  gained  by  the  swing-out  of  the  spectroscope  in  Fig.  141 
simple  instruments  like  Fig.  140  are  sometimes  mounted  with 
an  ordinary  ocular  in  a  double  nose-piece  like  that  used  for 
microscope  objectives,  so  that  either  can  be  used  at  will. 

Any  ordinary  pocket  spectroscope,  with  or  without  scale  or  a 
comparison  prism  over  part  of  the  slit,  can  in  fact  be  fitted  to  an 


Fig.  141. — Abbe  Ocular  Spectroscope. 


ACCESSORIES  185 

adapter  and  used  with  the  star  focussed  on  the  sht  and  a  cyhnd- 
rical  lens,  if  necessary,  as  an  eye-cap. 

Such  sht  spectroscopes  readily  give  the  characteristics  of 
stellar  spectra  and  those  of  the  brighter  nebulae  or  of  comets. 
They  enable  one  to  identify  the  more  typical  lines  and  compare 
them  with  terrestrial  sources,  and  save  for  solar  work  are  about 
all  the  amateur  observer  finds  use  for. 

For  serious  research  a  good  deal  more  of  an  instrument  is 
required,  with  a  large  telescope  to  collect  the  hght,  and  means  for 
photographing  the  spectra  for  permanent  record.  The  cumula- 
tive effect  of  prolonged  exposures  makes  it  possible  easily  to 
record  spectra  much  too  faint  to  see  with  the  same  aperture,  and 
exposures  are  often  extended  to  many  hours. 

Spectroscopes  for  such  use  commonly  employ  dense  flint  prisms 
of  about  60°  refracting  angle  and  refractive  index  of  about  1.65, 
one,  two,  or  three  of  these  being  fitted  to  the  instrument  as 
occasion  requires.  A  fine  example  by  Brashear  is  shown  in  Fig. 
142,  arranged  for  visual  work  on  the  24  inch  Lowell  refractor. 
Here  A  is  the  slit,  B  the  prism  box,  C  the  observing  telescope,  D 
the  micrometer  ocular  with  electric  lamp  for  illuminating  the 
wires,  and  E  the  link  motion  that  keeps  the  prism  faces  at  equal 
angles  with  collimator  and  observing  telescope  when  the  angle 
between  these  is  changed  to  observe  different  parts  of  the  spec- 
trum. This  precaution  is  necessary  to  maintain  the  best  of 
definition. 

When  photographs  are  to  be  taken  the  observing  telescope  is 
unscrewed  and  a  photographic  lens  and  camera  put  in  its  place. 
If  the  brightness  of  the  object  permits,  three  prisms  are  installed, 
turning  the  beam  180°  into  a  camera  braced  to  the  same  frame 
alongside  the  slit. 

For  purely  photographic  work,  too,  the  objective  prism  used 
by  Fraunhofer  for  the  earliest  observation  of  stellar  spectra  is  in 
wide  use.  It  is  a  prism  fitted  in  front  of  the  objective  with  its 
refracting  faces  making  equal  angles  with  the  telescope  and  the 
region  to  be  observed,  respectively.  Its  great  advantages  are 
small  loss  of  light  and  the  ability  to  photograph  many  spectra  at 
once,  for  all  the  stars  in  the  clear  field  of  the  instrument  leave 
their  images  spread  out  into  spectra  upon  the  photographic 
plate. 

Figure  143  shows  such  an  objective  prism  mounted  in  front  of 
an   astrographic   objective.     The   prism   is   rotatable   into   any 


186 


THE  TELESCOPE 


azimuth  about  the  axis  of  the  objective  and  by  the  scale  i  and 
clamping  screw  r  can  have  its  refracting  face  adjusted  with 


Fig.  142. — Typical  Stellar  Spectroscope. 

respect  to  that  axis  to  the  best  position  for  photographing  any 
part  of  the  spectrum.  Such  an  arrangement  is  typical  of  those 
used  for  small  instruments  say  from  3  inches  to  6  inches  aperture. 


ACCESSORIES 


187 


For  larger  objectives  the  prism  is  usually  of  decidedly  smaller 
angle,  and,  if  the  light  warrants  high  dispersion,  several  prisms 
in  tandem  are  used.  The  objective  prism  does  its  best  work  when 
applied  to  true  photographic  objectives  of  the  portrait  lens  type 
which  yield  a  fairly  large  field.  It  is  by  means  of  big  instruments 
of  such  sort  that  the  spectra  for  the  magnificent  Draper  Cata- 
logue have  been  secured  by  the 
Harvard  Observatory,  mostly  at  the 
Arequipa  station.  In  photographing 
with  the  objective  prism  the  spectra 
are  commonly  given  the  necessary 
width  for  convenient  examination  by 
changing  just  a  trifle  the  rate  of  the 
driving  clock  so  that  there  is  a  slight 
and  gradual  drift  in  R.  A.  The  re- 
fracting edge  of  the  prism  being  turned 
parallel  to  the  diurnal  motion  this  drift 
very  gradually  and  uniformly  widens 
the  spectrum  to  the  extent  of  a  few 
minutes  of  arc  during  the  whole 
exposure. 

When  one  comes  to  solar  spectro- 
scopy one  meets  an  entirely  different 
situation.  In  stellar  work  the  diffi- 
culty is  to  get  enough  light,  and  the 
tendency  is  toward  large  objectives 
of  relatively  short  focal  length  and 
spectroscopes  of  moderate  dispersion. 
In  solar  studies  there  is  ample  light, 
and  the  main  thing  is  to  get  an  image 
big  enough  to  be  scrutinized  in  detail  with  very  great  dispersion. 

Especially  is  this  true  in  the  study  of  the  chromospheric  flames 
that  rim  the  solar  disc  and  blaze  over  its  surface.  To  examine 
these  effectively  the  spectroscope  should  have  immense  dispersion 
with  a  minimum  amount  of  stray  light  in  the  field  to  interfere 
with  vision  of  delicate  details. 

In  using  a  spectroscope  like  Fig.  142,  if  one  turned  the  slit 
toward  the  landscape,  the  instrument  being  removed  from  the 
telescope  and  the  slit  opened  wide,  he  could  plainly  see  its  various 
features,  refracted  through  the  prism,  and  appearing  in  such 
color  as  corresponded  to  the  part  of  the  spectrum  in  the  line  of 


Fig.  143. 


-Simple  Objective 
Prism. 


188 


THE  TELESCOPE 


the  observing  telescope.  In  other  words  one  sees  refracted 
images  quite  distinctly  in  spite  of  the  bending  of  the  rays.  With 
high  dispersion  the  image  seen  is  practically  monochromatic. 

Now  if  one  puts  the  spectroscope  in  place,  brings  the  solar 
image  tangent  to  the  slit  and  then  cautiously  opens  the  slit,  he 
sees  the  bright  continuous  spectrum  of  the  sky  close  to  the  sun, 
plus  any  light  of  the  particular  color  for  which  the  observing 
telescope  is  set,  which  may  proceed  from  the  edge  of  the  solar 
disc.  Thus,  if  the  setting  is  for  the  red  line  of  hydrogen  (C),  one 
sees  the  hydrogen  glow  that  plays  in  fiery  pillars  of  cloud  about 


Fig.  144. — Diagram  of  Evershed  Solar  Spectroscope. 

the  sun's  limb  quite  plainly  through  the  opened  slit,  on  a  back- 
ground of  light  streaming  from  the  adjacent  sky.  To  see  most 
clearly  one  must  use  great  dispersion  to  spread  this  back-ground 
out  into  insignificance,  must  keep  other  stray  light  out  of  the 
field,  and  limit  his  view  to  the  opened  slit. 

To  these  ends  early  solar  spectroscopes  had  many  prisms  in 
tandem,  up  to  a  dozen  or  so,  kept  in  proper  relation  by  com- 
plicated linkages  analogous  to  the  simple  one  shown  in  Fig.  142. 
Details  can  be  found  in  almost  any  astronomical  work  of  40 
years  ago.  They  were  highly  effective  in  giving  dispersion  but 
neither  improved  the  definition  nor  cut  out  light  reflected  back 
and  forth  from  their  many  surfaces. 

Of  late  simpler  constructions  have  come  into  use  of  which  an 
excellent  type  is  the  spectroscope  designed  by  Mr.  Evershed  and 
shown  in  diagram  in  Fig.  144.  Here  the  path  of  the  rays  is  from 
the  slit  through  the  collimator  objective,  then  through  a  very 


ACCESSORIES  189 

powerful  direct  vision  system,  giving  a  dispersion  of  30°  through 
the  visible  spectrum,  then  by  reflection  from  the  mirror  through 
a  second  such  system,  and  thence  to  the  observing  telescope. 
The  mirror  is  rotated  to  get  various  parts  of  the  spectrum  into 
view,  and  the  micrometer  screw  that  turns  it  gives  means  for 
making  accurate  measurement  of  wave  lengths. 

There  are  but  five  reflecting  surfaces  in  the  prism  system  (for 
the  cemented  prism  surfaces  do  not  count  for  much)  as  against 
more  than  20  in  one  of  the  older  instruments  of  similar  power, 
and  as  in  other  direct  vision  systems  the  spectrum  lines  are 
substantially  straight  instead  of  being  strongly  curved  as  with 


Fig.   145.— Evershed  Solar  Spectroscope. 

multiple  single  prisms.  The  result  is  the  light,  compact,  and 
powerful  spectroscope  shown  complete  in  Fig.  145,  equally  well 
fitted  for  observing  the  sun's  prominences  and  the  detailed 
spectrum  from  his  surface. 

In  most  of  the  solar  spectroscopes  made  at  the  present  time 
the  prisms  are  replaced  by  a  diffraction  grating.  The  original 
gratings  made  by  Fraunhofer  were  made  of  wire.  Two  parallel 
screws  of  extremely  fine  thread  formed  two  opposite  sides  of  a 
brass  frame.  A  very  fine  wire  was  then  wound  over  these 
screws,  made  fast  by  solder  on  one  side  of  each,  and  then  cut 
away  on  the  other,  so  as  to  leave  a  grating  of  parallel  wires  with 
clear  spaces  between. 

Today  the  grating  is  generally  ruled  by  an  automatic  ruling 
engine  upon  a  polished  plate  of  speculum  metal.  The  diamond 
point  carried  by  the  engine  cuts  very  smooth  and  fine  parallel 
furrows,   commonly  from   10,000  to  20,000  to  the  inch.     The 


190  THE  TELESCOPE 

spaces  between  the  furrows  reflect  brilliantly  and  produce 
diffraction  spectra.^ 

When  a  grating  is  used  instead  of  prisms  the  instrument  is 
commonly  set  up  as  shown  in  Fig.  146.  Here  A  is  the  collimator 
with  slit  upon  which  the  solar  image  light  falls,  B  is  the  observing 
telescope,  and  C  the  grating  set  in  a  rotatable  mount  with  a  fine 
threaded  tangent  screw  to  bring  any  line  accurately  upon  the 
cross  wires  of  the  ocular. 

The  grating  gives  a  series  of  spectra  on  each  side  of  the  slit, 
violet  ends  toward  the  slit,  and  with  deviations  proportional  to 


Fig.  146. — Diagram  of  Grating  Spectroscope. 

1,  2,  3,  4,  etc.,  times  the  wave  length  of  the  line  considered.  The 
spectra  therefore  overlap,  the  ultra  violet  of  the  second  order 
being  superimposed  on  the  extreme  red  of  the  first  order  and  so 
on.  Colored  screens  over  the  slit  or  ocular  are  used  to  get  the 
overlying  spectra  out  of  the  way. 

The  grating  spectroscopes  are  very  advantageous  in  furnishing 
a  wide  range  of  available  dispersions,  and  in  giving  less  stray 
light  than  a  prism  train  of  equal  power.  The  spectra  moreover 
are  very  nearly  "normal,  "i.e.,  the  position  of  each  line  is  propor- 
tional to  its  wave  length  instead  of  the  blue  being  disproportion- 
ately long  as  in  prismatic  spectra. 

In  examining  solar  prominences  the  widened  slit  of  a  grating 
spectroscope  shows  them  foreshortened  or  stretched  to  an 
amount  depending  on  the  angular  position  of  the  grating,  but  the 
effect  is  easily  reckoned. ^ 

^For  the  principle  of  diffraction  spectra  see  Baly,  Spectroscopy;  Kayser, 
Handbuch  d.  Specktroskoie  or  any  of  the  larger  textbooks  of  physics. 

2  The  effect  on  the  observed  height  of  a  prominence  is  h    =   h'   -. — r' 

where  h  is  the  real  height,  h'  the  apparent  height,  c  the  angle  made  by  the 
grating  face  with  the  collimator,  and  t  that  with  the  telescope  (Fig.  146). 


ACCESSORIES  191 

If  the  slit  is  nearly  closed  one  sees  merely  a  thin  line,  irregularly 
bright  according  to  the  shape  of  the  prominence;  a  shift  of  the 
slit  with  respect  to  the  solar  image  shows  a  new  irregular  section 
of  the  prominence  in  the  same  monochromatic  light. 

These  simple  phenomena  form  the  basis  of  one  of  the  most 
important  instruments  of  solar  study — the  spectro-heliograph. 
This  was  devised  almost  simultaneously  by  G.  E.  Hale  and  M. 
Deslandres  about  30  years  ago,  and  enables  photographs  of  the 
sun  to  be  taken  in  monochromatic  light,  showing  not  only  the 
prominences  of  the  limb  but  glowing  masses  of  gas  scattered  all 
over  the  surface. 

The  principle  of  the  instrument  is  very  simple.  The  collimator 
of  a  powerful  grating  spectroscope  is  provided  with  a  slit  the  full 
length  of  the  solar  diameter,  arranged  to  slide  smoothly  on  a 
ball-bearing  carriage  clear  across  the  solar  disc.  Just  in  front  of 
the  photographic  plate  set  in  the  focus  of  the  camera  lens  is 
another  narrow  sliding  slit,  which,  like  a  focal  plane  shutter, 
exposes  strip  after  strip  of  the  plate. 

The  two  slits  are  geared  together  by  a  system  of  levers  or 
otherwise  so  that  they  move  at  exactly  the  same  uniform  rate  of 
speed.  Thus  when  the  front  slit  is  letting  through  a  monochro- 
matic section  of  a  prominence  on  the  sun's  limb  the  plate-slit  is 
at  an  exactly  corresponding  position.  When  the  front  slit  is 
exactly  across  the  sun's  center  so  is  the  plate  slit,  at  each  element 
of  movement  exposing  a  line  of  the  plate  to  the  monochromatic 
image  from  the  moving  front  slit.  The  grating  can  of  course  be 
turned  to  put  any  required  line  into  action  but  it  usually  is  set 
for  the  K  line  (calcium),  which  is  photographically  very  brilliant 
and  shows  bright  masses  of  floating  vapor  all  over  the  sun's 
surface. 

Figure  147  shows  an  early  and  simple  type  of  Professor  Hale's 
instrument.  Here  A  is  the  collimator  with  its  sliding  slit,  B  the 
photographic  telescope  with  its  corresponding  slide  and  C  the 
lever  system  which  connects  the  slides  in  perfectly  uniform 
alignment.  The  source  of  power  is  a  very  accurately  regulated 
water  pressure  cylinder  mounted  parallel  with  the  collimator. 
The  result  is  a  complete  photograph  of  the  sun  taken  in  mono- 
chromatic light  of  exactly  defined  wave  length  and  showing  the 
precise  distribution  of  the  glowing  vapor  of  the  corresponding 
substance. 

Since  the  spectroheliograph  of  Fig.  147,  which  shows  the  princi- 


192 


THE  TELESCOPE 


pie  remarkably  well,  there  have  been  made  many  modifications, 
in  particular  for  adapting  the  scheme  to  the  great  horizontal  and 
vertical  fixed  telescopes  now  in  use.  (For  details  of  these  see 
Cont.  from  the  Solar  Obs.  Mt.  Wilson,  Nos.  3,  4,  23,  and  others). 
The  chief  difficulty  always  is  to  secure  entirely  smooth  and 
uniform  motion  of  the  two  moving  elements. 


Fig.  147. — Hale's  Spectroheliograph  (Early  Form). 

So  great  and  interesting  a  branch  of  astronomy  is  the  study  of 
variable  stars  that  some  form  of  photometer  should  be  part  of 
the  equipment  of  every  telescope  in  serious  use  for  celestial 
observation.  An  immense  amount  of  useful  work  has  been  done 
by  Argelander's  systematic  method  of  eye  observation,  but  it  is 
far  from  being  precise  enough  to  disclose  many  of  the  most 
important  features  of  variability. 


ACCESSORIES 


193 


The  conventional  way  of  reckoning  by  stellar  magnitudes  is 
conducive  to  loose  measurements,  since  each  magnitude  of 
difference  implies  a  light  ratio  of  which  the  log  is  0.4,  i.e.,  each 
magnitude  is  2.512  times  brighter  than  the  following  one.  As  a 
result  of  this  way  of  reckoning  the  light  of  a  star  of  mag.  9.9  differs 
from  one  of  mag.  10.0  not  by  one  per  cent  but  by  about  nine. 
Hence  to  grasp  light  variations  of  small  order  one  must  be  able  to 
measure  far  below  O'^l. 

The  ordinary  laboratory  photometer  enables  one  to  compare 
light  sources  of  anywhere  near  similar  color  to  a  probable  error 


Fig.  148. — Double  Image  Stellar  Photometer. 


of  well  under  0.1  per  cent,  but  it  allows  a  comparison  between 
sharply  defined  juxtaposed  fields  from  the  two  illuminants,  a 
condition  much  more  favorable  to  precision  than  the  comparison 
of  two  points  of  light,  even  if  fairly  near  together. 

Stellar  photometers  may  in  principle  be  divided  into  three 
classes.  (1)  Those  in  which  two  actual  stars  are  brought  into  the 
same  field  and  compared  by  varying  the  light  from  one  or  both  in 
a  known  degree.  (2)  Those  which  bring  a  real  star  into  the 
field  alongside  an  artificial  star,  and  again  bring  the  two  to 
equality  by  a  known  variation,  usually  comparing  two  or  more 
stars  via  the  same  artificial  star;  (3)  those  which  measure  the 
light  of  a  star  by  a  definite  method  of  extinguishing  it  entirely  or 
just  to  the  verge  of  disappearance  in  a  known  progression.  Of 
each  class  there  are  divers  varieties.  The  type  of  the  first  class 
may  be  taken  as  the  late  Professor  E.  C.  Pickering's  polarizing 
photometer.  Its  optical  principle  is  shown  in  Fig.  148.  Here 
the  brightness  of  two  neighboring  objects  is  compared  by  polariz- 
ing at  90°  apart  the  light  received  from  each  and  reducing  the 
resulting  images  to  equality  by  an  analyzing  Nicol  prism.     The 

13 


194  THE  TELESCOPE 

photometer  is  fully  described,  with,  several  other  polarizing 
instruments,  in  H.  A.  Vol.  II  from  which  Fig.  148  is  taken. 

A  is  a  Nicol  prism  inserted  in  the  ocular  5,  which  revolves  carry- 
ing with  it  a  divided  circle  C  read  against  the  index  D.  In  the 
tube  E  which  fits  the  eye  end  of  the  telescope,  is  placed  the  double 
image  quartz  prism  F  capable  of  sliding  either  way  without 
rotation  by  pulling  the  cord  G.  With  the  objects  to  be  compared 
in  the  same  field,  two  images  of  each  appear.  By  turning  the 
analyzing  Nicol  the  fainter  image  of  the  brighter  can  always  be 
reduced  to  equality  with  the  brighter  image  of  the  fainter,  and  the 
amount  of  rotation  measures  the  required  ratio  of  brightness.  ^ 
This  instrument  works  well  for  objects  near  enough  to  be  in  the 
same  field  of  view.  The  distance  between  the  images  can  be 
adjusted  by  sliding  the  prism  F  back  and  forth,  but  the  available 
range  of  view  is  limited  to  a  small  fraction  of  a  degree  in  ordinary 
telescopes. 

The  meridian  photometer  was  designed  to  avoid  this  small 
scope.  The  photometric  device  is  substantially  the  same  as 
in  Fig.  148.  The  objects  compared  are  brought  into  the  field 
by  two  exactly  similar  objectives  placed  at  a  small  angle  so  that 
the  images,  after  passing  the  double  image  prism,  are 
substantially  in  coincidence.  In  front  of  each  of  the  objectives  is 
a  mirror.  The  instrument  points  in  the  east  and  west  line  and  the 
mirrors  are  at  45°  with  its  axis.  One  brings  Polaris  into  the  field, 
the  other  by  a  motion  of  rotation  about  the  telescope  axis  can 
bring  any  object  in  or  close  to  the  meridian  into  the  field  along- 
side Polaris.  The  images  are  then  compared  precisely  as  in  the 
preceding  instance.^  There  are  suitable  adjustments  for  bringing 
the  images  into  the  positions  required. 

The  various  forms  of  photometer  using  an  artificial  star  as 
intermediary  in  the  comparison  of  real  stars  differ  chiefly  in  the 

^  If  A  be  the  brightness  of  one  object  and  B  that  of  the  other,  a  the  reading 
of  the  index  when  one  image  disappears  and  /3  the  reading  when  the  two 

A 
images  are  equal  then  :„  =  tan^  (a  —  /3).     There  are  four  positions  of  the 

Nicol,  90°  apart,  for  which  equality  can  be  established,  and  usually  all  are 
read  and  the  mean  taken.     (H.  A.  II,  1.) 

2  For  full  description  and  method  see  H.  A.  Vol.  14,  also  Miss 
Furness'  admirable  " Introduction  to  the  Study  of  Variable  Stars,"  p.  122, 
etseq.  Some  modifications  are  described  in  H.  A.  Vol.  23.  These  direct 
comparison  photometers  give  results  subject  to  some  annoying  small  correc- 
tions, but  a  vast  amount  of  valuable  work  has  been  done  with  them  in  the 
Harvard  Photometry. 


ACCESSORIES 


195 


method  of  varying  the  Kght  in  a  determinate  measure.  Rather 
the  best  known  is  the  ZoUner  instrument  shown  in  diagram  in 
Fig.  149.  Here  A  is  the  eye  end  of  the  main  telescope  tube. 
Across  it  at  an  angle  of  45°  is  thrown  a  piece  of  plane  parallel 
glass  B  which  serves  to  reflect  to  the  focus  the  beam  from  down 
the  side  tube,  C,  forming  the  artificial  star. 


Fig.  149. — ZoUner  Photometer  Diagram. 

At  the  end  of  this  tube  is  a  small  hole  or  more  often  a 
diaphragm  perforated  with  several  very  small  holes  any  of 
which  can  be  brought  into  the  axis  of  the  tube.  Beyond  at  D, 
is  the  source  of  light,  originally  a  lamp  flame,  now  generally  a 
small  incandescent  lamp,  with  a  ground  glass  disc  or  surface 
uniformly  to  diffuse  the  light. 

Within  the  tube  C  lie  three  Nicol  prisms  n,  ni,  nz.  Of  these  n, 
is  fixed  with  respect  to  the  mirror  B  and  forms  the  analyser, 
which  Hi  and  W2  turn  together  forming  the  polarizing  system. 


196 


THE  TELESCOPE 


Between  rii  and  n^  is  a  quartz  plate  e  cut  perpendicular  to  the 
crystal  axis.  The  color  of  the  light  transmitted  by  such  a  plate 
in  polarized  light  varies  through  a  wide  range.  By  turning  the 
Nicol  W2  therefore,  the  color  of  the  beam  which  forms  the  arti- 
ficial star  can  be  made  to  match  the  real  star  under  examination, 
and  then  by  turning  the  whole  system  n^,  E,  ni,  reading  the 
rotation  on  the  divided  circle  at  F,  the  real  star  can  be  matched 
in  intensity  by  the  artificial  one. 


Fig.  150. — Wedge  Photometer. 


This  is  viewed  via  the  lens  G  and  two  tiny  points  of  light 
appear  in  the  field  of  the  ocular  due  respectively  to  reflection  from 
the  front  and  back  of  the  mirror  B,  the  latter  slightly  fainter 
than  the  former.  Alongside  or  between  these  the  real  star  image 
can  be  brought  for  a  comparison,  and  by  turning  the  polarizer 
through  an  angle  a  the  images  can  be  equalized  with  the  real 
image.  Then  a  similar  comparison  is  made  with  a  reference 
star.    If  A  be  the  brightness  of  the  former  and  B  of  the  latter  then 


A 
B 


sin^  a 
sin^  j8 


ACCESSORIES 


197 


The  Zollner  photometer  was  at  first  set  up  as  an  alt-azimuth 
instrument  with  a  small  objective  and  rotation  in  altitude  about 
the  axis  C.  Since  the  general  use  of  electric  lamps  instead  of  the 
inconvenient  flame  it  is  often  fitted  to  the  eye  end  of  an  equatorial. 
Another  very  useful  instrument  is  the  modern  wedge  photo- 
meter, closely  resembling  the  Zollner  in  some  respects  but  with  a 
very  different  method  of  varying  the  light;  devised  by  the  late 
Professor  E.  C.  Pickering.  It  is  shown  somewhat  in  diagram  in 
Fig.  150.  Here  as  before  O  is  the  eye  end  of  the  tube,  B  the  plane 
parallel  reflector,  C  the  side  tube,  L  the  source  of  light  D  the 
diaphragm  and  A  the  lens  forming  the  artificial  star  by  projecting 


sj 


Fig.  151. — Simple  Polarizing  Photometer. 


the  hole  in  the  diaphragm.  In  actual  practice  the  diameter  of 
such  hole  is  j^f  oo  inch  or  less. 

The  light  varying  device  W  is  a  "photographic  wedge"  set  in  a 
frame  which  is  graduated  on  the  edge  and  moved  in  front  of  the 
aperture  by  a  rack  and  pinion  at  R.  There  are  beside  colored 
and  shade  glasses  for  use  as  occasion  requires.  The  "photo- 
graphic wedge"  is  merely  a  strip  of  fine  grained  photographic 
plate  given  an  evenly  graduated  exposure  from  end  to  end, 
developed,  and  sealed  under  a  cover  glass.  Its  absorption  is 
permanent,  non-selective  as  to  color,  and  it  can  be  made  to  shade 
off  from  a  barely  perceptible  density  to  any  required  opacity. 
Sometimes  a  wedge  of  neutral  tinted  glass  is  used  in  its  stead. 

Before  using  such  a  "wedge  photometer"  the  wedge  must  be 
accurately  calibrated  by  observation  of  real  or  artificial  stars  of 
known  difference  in  brightness.  This  is  a  task  demanding  much 
care  and  is  well  described,  together  with  the  whole  instrument 


198  THE  TELESCOPE 

by  Parkhurst  (Ap.  J.  13,  249).  The  great  difficulty  with  all 
instruments  of  this  general  type  is  the  formation  of  an  artificial 
star  the  image  of  which  shall  very  closely  resemble  the  image 
of  the  real  star  in  appearance  and  color. 

•  Obviously  either  the  real  or  artificial  star,  or  both,  may  be 
varied  in  intensity  by  wedge  or  Nicols,  and  a  very  serviceable 
modification  of  the  Zollner  instrument,  with  this  in  mind  was 
recently  described  by  Shook  (Pop.  Ast.  27,  595)  and  is  shown 
in  diagram  in  Fig.  151.  Here  A  is  the  tube  which  fits  the  ordinary 
eyepiece  sleeve.  E  is  a  side  tube  into  which  is  fitted  the  extension 
D  with  a  fitting  H  at  its  outer  end  into  which  sets  the  lamp 
tube  G.  This  carries  on  a  base  plug  F  a  small  flash  light  bulb 
run  by  a  couple  of  dry  cells.  At  0  is  placed  a  little  brass  dia- 
phragm perforated  with  a  minute  hole.  Between  this  and  the 
lamp  is  a  disc  of  diffusing  glass  or  paper.  A  Nicol  prism  is  set 
a  little  ahead  of  0,  and  a  lens  L  focusses  the  perforation  at  the 
principal  focus  of  the  telescope  after  reflection  from  the  diagonal 
glass  M,  as  in  the  preceding  examples.  I  is  an  ordinary  eyepiece 
over  which  is  a  rotatable  Nicol  N  with  a  position  circle  K. 
At  P  is  a  third  Nicol  in  the  path  of  the  rays  from  the  real  star, 
thereby  increasing  the  convenient  range  of  the  instrument. 
The  original  paper  gives  the  details  of  construction  as  well  as 
the  methods  of  working.  Obviously  the  same  general  arrange- 
ment could  be  used  for  a  wedge  photometer  using  the  wedge  on 
either  real  or  artificial  star  or  both. 

The  third  type  of  visual  photometer  depends  on  reducing  the 
light  of  the  star  observed  until  it  just  disappears.  This  plan 
was  extensively  employed  by  Professor  Pritchard  of  Oxford  some 
40  years  ago.  He  used  a  sliding  wedge  of  dark  glass,  carefully 
calibrated,  and  compared  two  stars  by  noting  the  point  on  the 
wedge  at  which  each  was  extinguished,  A  photographic  wedge 
may  be  used  in  exactly  the  same  way. 

Another  device  to  the  same  end  depends  on  reducing  the 
aperture  of  the  telescope  by  a  "cat's  eye,"  an  iris  diaphragm,  or 
similar  means  until  the  star  is  no  longer  visible  or  just  disap- 
pearing. The  great  objection  to  such  methods  is  the  extremely 
variable  sensitivity  of  the  eye  under  varying  stimulus  of  light. 

The  most  that  can  be  said  for  the  extinction  photometer  is 
that  in  skillful  and  experienced  hands  like  Pritchard's  it  has 
sometimes  given  much  more  consistent  readings  than  would  be 
expected.     It  is  now  and  then  very  convenient  for  quick  approxi- 


ACCESSORIES  199 

mation  but  by  no  courtesy  can  it  be  considered  an  instrument  of 
precision  either  in  astronomical  or  other  photometry.  ^ 

The  photometer  question  should  not  be  closed  without  referring 
the  reader  to  the  methods  of  physical  photometry  as  developed  by 
Stebbins,  Guthnick  and  others.  The  first  of  these  depends  on 
the  use  of  the  selenium  cell  in  which  the  electrical  resistance  falls 
on  exposure  of  the  selenium  to  light.  The  device  is  not  one 
adapted  to  casual  use,  and  requires  very  careful  nursing  to  give 
the  best  results,  but  these  are  of  an  order  of  precision  beyond 
anything  yet  reached  with  an  astronomical  visual  photometer. 
Settings  come  down  to  variations  of  something  hke  2  per  cent,  and 
variations  in  stellar  light  entirely  escaping  previous  methods 
become  obvious. 

The  photoelectric  cell  depends  on  the  lowering  of  the  apparent 
electric  resistance  of  a  layer  of  rarified  inert  gas  between  a  plati- 
num grid  and  an  electrode  of  metallic  potassium  or  other  alkali 
metal  when  light  falls  on  that  electrode.  The  rate  of  transmission 
of  electricity  is  very  exactly  proportional  to  the  illumination, 
and  can  be  best  measured  by  a  very  sensitive  electrometer. 
The  results  are  extraordinarily  consistent,  and  the  theoretical 
"probable  error"  is  very  small,  though  here,  as  elsewhere, 
"probable  error"  is  a  rather  meaningless  term  apt  to  lead  to  a 
false  presumption  of  exactness.  Again  the  apparatus  is  somewhat 
intricate  and  delicate,  but  gives  a  precision  of  working  if  any- 
thing a  little  better  than  that  of  the  selenium  cell,  quite  certainly 
below  1  per  cent. 

Neither  instrument  constitutes  an  attachment  to  the  ordinary 
telescope  of  modest  size  which  can  be  successfully  used  for  ordi- 
nary photometry,  and  both  require  reduction  of  results  to  the 
basis  of  visual  effect.^     But  both  offer  great  promise  in  detecting 

^  The  general  order  of  precision  attained  by  astronomical  photometers  is 
shown  in  the  discovery,  photographically,  by  Hertzsprung  in  1911,  that 
Polaris,  used  as  a  standard  magnitude  for  many  years,  is  actually  a  variable. 
Its  period  is  very  near  to  four  days,  its  photographic  amplitude  0.17  and  its 
visual  amplitude  about  0.1,  i.e.,  a  variation  of  ±  5  per  cent  in  the  light  was 
submerged  in  the  observational  uncertainties,  although  once  known  it  was 
traced  out  in  the  accumulated  data  without  great  difficulty. 

^  Such  apparatus  is  essentially  appurtenant  to  large  instruments  only,  say 
of  not  less  than  12"  aperture  and  preferably  much  more.  The  eye  is 
enormously  more  sensitive  as  a  detector  of  radiant  energy  than  any  device  of 
human  contrivance,  and  thus  small  telescopes  can  be  well  used  for  visual 
photometry,  the  bigger  instruments  having  then  merely  the  advantage  of 
reaching  fainter  stars. 


200  "i'JIE  TELESCOPE 

minute  variations  of  light  and  have  done  admirable  work.  For  a 
good  fundamental  description  of  the  selenium  cell  photometer  see 
Stebbins,  Ap.  J.  32,  185  and  for  the  photoelectric  method  see 
Guthnick  A.  N.  196,  357  also  A.  F.  and  F.  A.  Lindemann, 
M.  N.  39,  343.  The  volume  by  Miss  Furness  already  referred 
to  gives  some  interesting  details  of  both. 


CHAPTER  IX 
THE  CARE  AND  TESTING  OF  TELESCOPES 

A  word  at  the  start  concerning  the  choice  and  purchase  of 
telescopes.  The  question  of  refractors  vs.  reflectors  has  been 
already  considered.  The  outcome  of  the  case  depends  on  how 
much  and  how  often  you  are  likely  to  use  the  instrument,  and 
just  what  you  want  it  for.  For  casual  observations  and  occa- 
sional use — all  that  most  busy  buyers  of  telescopes  can  expect — 
the  refractor  has  a  decided  advantage  in  convenience.  If  one 
has  leisure  for  frequent  observations,  and  particularly  if  he  can 
give  his  telescope  a  permanent  mount,  and  is  going  in  for  serious 
work,  he  will  do  well  not  to  dismiss  the  idea  of  a  reflector  without 
due  deliberation. 

In  any  case  it  is  good  policy  to  procure  an  instrument  from  one 
of  the  best  makers.  And  if  you  do  not  buy  directly  of  the  actual 
maker  it  is  best  to  deal  with  his  accredited  agents.  In  other 
words  avoid  telescopes  casuallj^  picked  up  in  the  optical  trade 
unless  you  chance  to  have  facilities  for  thorough  testing  under 
competent  guidance  before  purchase.  No  better  telescopes  are 
made  than  can  be  had  from  the  best  American  makers.  A  few 
British  and  German  makers  are  quite  in  the  same  class.  So  few 
high  grade  French  telescopes  reach  this  country  as  to  cause  a 
rather  common,  but  actually  unjust,^  belief  that  there  are  none. 

If  economy  must  be  enforced  it  is  much  wiser  to  try  to  pick  up  a 
used  instrument  of  first  class  manufacture  than 'to  chance  a  new- 
one  at  a  low  price.  Now  and  then  a  maker  of  very  ordinary 
repute  may  turn  out  a  good  instrument,  but  the  fact  is  one  to  be 
proved — not  assumed.  Age  and  use  do  not  seriously  deteriorate 
a  telescope  if  it  has  been  given  proper  care.  Some  of  Fraun- 
hofer's  are  still  doing  good  service  after  a  century,  and  occasion- 
ally an  instrument  from  one  of  the  great  makers  comes  into  the 
market  at  a  real  bargain.  It  may  drift  back  to  the  maker  for 
resale,  or  turn  up  at  any  optician's  shop,  and  in  any  case  is  better 
worth  looking  at  than  an  equally  cheap  new  telescope. 

1  E.  g.,  the  beautiful  astrographic  and  other  objectives  turned  out  by 
the  brothers  Henry. 

201 


202  THE  TELESCOPE 

The  condition  of  the  tube  and  stand  cuts  httle  figure  if  they  are 
mechanically  in  good  shape.  Most  of  the  older  high  grade 
instruments  were  of  brass,  beautifully  finished  and  lacqliered,  and 
nothing  looks  worse  after  hard  usage.  It  is  essential  that  the 
fitting  of  the  parts  should  be  accurate  and  that  the  focussing  rack 
should  work  with  the  utmost  smoothness.  A  fault  just  here, 
however,  can  be  remedied  at  small  cost.  The  mount,  whatever 
its  character,  should  be  likewise  smooth  working  and  without  a 
trace  of  shakiness,  unless  one  figures  on  throwing  it  away. 

As  to  the  objective,  it  demands  very  careful  examination 
before  a  real  test  of  its  optical  qualities.  The  objective  with  its 
cell  should  be  taken  out  and  closely  scrutinized  in  a  strong  light 
after  the  superficial  dust  has  been  removed  with  a  camel's 
hair  brush  or  by  wiping  very  gently  with  the  soft  Japanese  "lens 
paper"  used  by  opticians. 

One  is  likely  to  find  plenty  to  look  at;  spots,  finger  marks, 
obvious  scratches,  and  what  is  worse  a  net-work  of  superficial 
scratches,  or  a  surface  with  patches  looking  like  very  fine  pitting. 
These  last  two  defects  imply  the  need  of  repolishing  the  affected 
surface,  which  means  also  more  or  less  refiguring.  Ordinary 
brownish  spots  and  finger  marks  can  usually  be  removed  with 
little  trouble. 

The  layman,  so  to  speak,  is  often  warned  never  to  remove  the 
cell  from  a  telescope  but  he  might  as  well  learn  the  simpler 
adjustments  first  as  last.  In  taking  off  a  cell  the  main  thing  is  to 
see  what  one  is  about  and  to  proceed  in  an  orderly  manner. 
If  the  whole  cell  unscrews,  as  often  is  the  case  in  small  in- 
struments, the  only  precaution  required  is  to  put  a  pencil  mark 
on  the  cell  and  its  seat  so  that  it  can  be  screwed  back  to  where 
it  started. 

If  as  is  more  usual  the  cell  fits  on  with  three  pairs  of  screws,  one 
of  each  pair  will  form  an  abutment  against  which  its  mate  pulls 
the  cell.  A  pencil  mark  locating  the  position  of  the  head  of  each 
of  the  pulling  screws  enables  one  to  back  them  out  and  replace 
them  without  shifting  the  cell. 

The  first  inspection  will  generally  tell  whether  the  objective  is 
worth  further  trouble  or  not.  If  all  surfaces  save  the  front  are  in 
good  condition  it  may  pay  to  send  the  objective  to  the  maker  for 
repolishing.  If  more  than  one  surface  is  in  bad  shape  reworking 
hardly  pays  unless  the  lens  can  be  had  for  a  nominal  figure.  In 
buying  a  used  instrument  from  its  original  source  these  precau- 


THE  CARE  AND  TESTING  OF  TELESCOPES  203 

tions  are  needless  as  the  maker  can  be  trusted  to  stand  back  of 
his  own  and  to  put  it  in  first  class  condition. 

However,  granted  that  the  objective  stands  well  the  inspection 
for  superficial  defects,  it  should  then  be  given  a  real  test  for 
figure  and  color  correction,  bearing  in  mind  that  objectives,  even 
from  first  class  makers,  may  now  and  then  show  slightly  faulty 
corrections,  while  those  from  comparatively  unknown  sources 
may  now  and  then  turn  out  well.  In  this  matter  of  necessary 
testing  old  and  new  glasses  are  quite  on  all  fours  save  that  one 
may  safely  trust  the  maker  with  a  well  earned  reputation  to  make 
right  any  imperfections.  Cleansing  other  than  dusting  off  and 
cautiously  wiping  with  damp  and  then  dry  lens  paper  requires 
removal  of  the  lenses  from  their  cell  which  demands  real  care. 

With  a  promising  looking  objective,  old  or  new,  the  first  test 
to  be  applied  is  the  artificial  star — artificial  rather  than  natural 
since  the  former  stays  put  and  can  be  used  by  day  or  by  night. 
For  day  use  the  "star"  is  merely  the  bright  reflection  of  the  sun 
from  a  sharply  curved  surface — the  shoulder  of  a  small  round 
bottle,  a  spherical  flask  silvered  on  the  inside,  a  small  silvered 
ball  such  as  is  used  for  Christmas  tree  decoration,  a  bicycle  ball, 
or  a  glass  "alley"  dear  to  the  heart  of  the  small  boy. 

The  object,  whatever  it  is,  should  be  set  up  in  the  sun  against 
a  dark  background  distant  say  40  or  50  times  the  focal  length  of 
the  objective  to  be  tested.  The  writer  rather  likes  a  silvered  ball 
cemented  to  a  big  sheet  of  black  cardboard.  At  night  a  pin 
hole  say  3'i2  inch  or  less  in  diameter  through  cardboard  or 
better,  tinfoil,  with  a  flame,  or  better  a  gas  filled  incandescent 
lamp  behind  it,  answers  well.  The  latter  requires  rather  careful 
adjustment  that  the  projected  area  of  the  closely  coiled  little 
filament  may  properly  fill  the  pinhole  just  in  front  of  it. 

Now  if  one  sets  up  the  telescope  and  focusses  it  approximately 
with  a  low  power  the  star  can  be  accurately  centered  in  the  field. 
Then  if  the  eyepiece  is  removed,  the  tube  racked  in  a  bit,  and 
the  eye  brought  into  the  focus  of  the  objective,  one  can  inspect 
the  objective  for  striae.  If  these  are  absent  the  field  will  be 
uniformly  bright  all  over.  Not  infrequently  however  one  will  see 
a  field  like  Fig.  152  or  Fig.  153.  The  former  is  the  appearance  of  a 
4  inch  objective  that  the  author  recently  got  his  eye  upon.  The 
latter  shows  typical  striae  of  the  ordinary  sort.  An  objective  of 
glass  as  bad  as  shown  in  Fig.  152  gives  no  hope  of  astronomical 
usefulness,  and  should  be  relegated  to  the  porch  of  a  seashore 


204 


THE  TELESCOPE 


cottage.     Figure    153    may    represent    a    condition    practically 
harmless  though  undesirable. 

The  next  step  is  a  really  critical  examination  of  the  focal 
image.  Using  a  moderately  high  power  ocular,  magnifying  say 
50  to  the  inch  of  aperture,  the  star  should  be  brought  to  the 
sharpest  focus  possible  and  the  image  closely  examined.  If  the 
objective  is  good  and  in  adjustment  this  image  should  be  a  very 
small  spot  of  light,  perfectly  round,  softening  very  slightly  in  its 


Fig.  152.— a  Bad  Case  of  Striae,^  ^^U^vyt^jG^lSS.— Ordinary  Strise. 

brilliancy  toward  the  edge,  and  surrounded  by  two  or  three  thin, 
sharp,  rings  of  light,  exactly  circular  and  with  well  defined  dark 
spaces  separating  them. 

Often  from  the  trembling  of  the  air  the  rings  will  seem  shaky 
and  broken,  but  still  well  centered  on  the  star-disc.  The  general 
appearance  is  that  shown  in  Fig.  154.^ 

Instead,  several  very  different  appearances  may  turn  up. 
First,  the  bright  diffraction  rings  may  be  visible  only  on  one 
side  of  the  central  disc,  which  may  itself  be 
drawn  out  in  the  same  direction.  Second, 
the  best  image  obtainable  may  be  fairly 
sharp  but  angular  or  irregular  instead  of 
round  or  oval  and  perhaps  with  a  hazy 
flare  on  one  side.  Third,  it  may  be  im- 
possible to  get  a  really  sharp  focus  any- 
where, the  image  being  a  mere  blob  of  light  with  nothing  definite 
about  it. 

1  This  and  several  of  the  subsequent  figures  are  taken  from  quite  the  best 
account  of  testing  objectives :  "  On  the  Adjustment  and  Testing  of  Telescope 
Objectives."  T.  Cooke  &  Sons,  York,  1891,  a  little  brochure  unhappily- 
long  since  out  of  print.     A  new  edition  is  just  now,  1922,  announced. 


Fig 


154. — A  First  Class 
Star  Image. 


THE  CARE  AND  TESTING  OF  TELESCOPES 


205 


Fig.  155.— Effect  of 
Objective  Askew. 


One  should  be  very  sure  that  the  eye-piece  is  clean  and  without 
fault  before  proceeding  further.  As  to  the  first  point  a  bit  of 
lens  paper  made  into  a  tiny  swab  on  a  sliver  of  soft  wood  will  be 
of  service,  and  the  surfaces  should  be  inspected  with  a  pocket 
lens  in  a  good  light  to  make  sure  that  the  cleaning  has  been 
thorough.  Turning  the  ocular  round  will  show  whether  any 
apparent  defects  of  the  image  turn  with  it. 

In  the  first  case  mentioned  the  next  step  is  to  rack  the  ocular 
gently  out  when  the  star  image  will  expand  into  a  more  or  less 
concentric  series  of  bright  interference 
rings  separated  by  dark  spaces,  half  a  dozen 
or  so  resulting  from  a  rather  small  move- 
ment out  of  focus.  If  these  rings  are  out 
of  round  and  eccentric  like  Fig.  155  one 
has  a  clear  case  of  failure  of  the  objective 
to  be  square  with  the  tube,  so  that  the 
ocular  looks  at  the  image  askew. 

in  the  ordinary  forms  of  objective  this  means  that  the  side  of 
the  objective  toward  the  brighter  and  less  expanded  part  of  the 
ring  system  is  too  near  the  ocular.  This  can  be  remedied  by 
pushing  that  side  of  the  objective  outwards  a  trifle.  Easing  off 
the  pulling  screw  on  that  side  and  slightly  tightening  the  abut- 
ment screw  makes  the  needed  correction,  which  can  be  lessened 
if  over  done  at  the  first  trial,  until  the  ring  system  is  accurately 
centered.  It  is  a  rather  fussy  job  but  not  at  all  difficult  if  one 
remembers  to  proceed  cautiously  and  to  use  the  screw  driver 
gently. 

In  the  second  case,  racking  out  the  ocular  a  little  gives  a  ring 
system  which  exaggerates  just  the  defects  of 
the  image.  The  faults  may  be  due  to  me- 
chanical strain  of  the  objective  in  its  cell, 
which  is  easily  cured,  or  to  strains  or  fiaws  in 
the  glass  itself,  which  are  irremediable. 
Therefore  one  should,  with  the  plane  of  the 
objective  horizontal,  loosen  the  retaining  ring 
that  holds  the  lenses,  without  disturbing  them, 
and  then  set  it  back  in  gentle  contact  and  try  the  out  of  focus 
rings  once  more.  If  there  is  no  marked  improvement  the  fault 
lies  in  the  glass  and  no  more  time  should  be  wasted  on  that 
particular  objective.  Fig.  156  is  a  typical  example  of  this  fault. 
In  dealing  with  case  three  it  is  well  to  give  the  lens  a  chance  by 


Fig.  156.— Effect  of 
Flaws  in  Objective. 


206  THE  TELESCOPE 

relieving  it  of  any  such  mechanical  strains,  for  now  and  then 
they  will  apparently  utterly  ruin  the  definition,  but  the  prognosis 
is  very  bad  unless  the  objective  has  been  most  brutally 
mishandled. 

In  any  case  failure  to  give  a  sharply  defined  focus  in  a  very 
definite  plane  is  a  warning  that  the  lens  (or  mirror)  is  rather  bad. 
In  testing  a  reflector  some  pains  must  be  taken  at  the  start  with 
both  the  main  and  the  secondary  mirror.  Using  an  artificial 
star  as  before,  one  should  focus  and  look  sharply  to  the  symmetry 
of  the  image,  taking  care  to  leave  the  instrument  in  observing 
position  and  screened  from  the  sun  for  an  hour  or  two  before 
testing.  Reflectors  are  much  more  sensitive  to  temperature  than 
refractors  and  take  longer  to  settle  down  to  stability  of  figure. 
With  a  well  mounted  telescope  of  either  sort  a  star  at  fair  altitude 
on  a  fine  night  gives  even  better  testing  conditions  than  an 
artificial  star,  (Polaris  is  good  in  northern  latitudes)  but  one  may 
have  a  long  wait. 

If  the  reflector  is  of  good  figure  and  well  adjusted,  the  star 
image,  in  focus  or  out,  has  quite  the  same  appearance  as  in  a 
refractor  except  that  with  a  bright  star  in  focus  one  sees  a  thin 
sharp  cross  of  light  centered  on  the  image,  rather  faint  but 
perfectly  distinct.  This  is  due  to  the  diffraction  effect  of  the 
four  thin  strips  that  support  the  small 
mirror,  and  fades  as  the  star  is  put  out  of 
focus. 

The  rings  then  appear  as  usual,  but  also 
a  black  disc  due  to  the  shadowing  of  the 
small  mirror.  Fig.  157  shows  the  extra- 
focal  image  of  a  real  or  artificial  star  when 

Fig    157 Extra-focal 

Image  from  Reflector,  the  mirror  is  well  Centered,  and  the  star  in 
the  middle  of  the  field.  There  only  are 
the  rings  round  and  concentric  with  the  mirror  spot.  The  rings 
go  out  of  round  and  the  spot  out  of  center  for  very  small  departure 
from  the  middle  of  the  field  when  the  mirror  is  of  large  relative 
aperture — F/5  or  F/6. 

If  the  star  image  shows  flare  or  oval  out-of-focus  rings  when 
central  of  the  field,  one  or  both  mirrors  probably  need  adjust- 
ment. Before  laying  the  trouble  to  imperfect  figure,  the  mirrors 
should  be  adjusted,  the  small  one  first  as  the  most  likely  source 
of  trouble.  The  side  of  the  mirror  toward  which  the  flare  or  the 
expanded  side  of  the  ring  system  projects  should  be  slightly 


THE  CARE  AND  TESTING  OF  TELESCOPES  207 

pushed  away  from  the  ocular.  (Note  that  owing  to  the  reflec- 
tion this  movement  is  the  reverse  of  that  required  with  a 
refractor.) 

If  the  lack  of  symmetry  persists  one  may  as  well  get  down  to 
first  principles  and  center  the  mirrors  at  once.  Perhaps  the 
easiest  plan  is  to  prepare  a  disc  of  white  card-board  exactly  the 
size  of  the  mirror  with  concentric  circles  laid  out  upon  it  and  an 
eighth  inch  hole  in  the  center.  Taking  out  the  ocular  and  putting 
a  half  inch  stop  in  its  place  one  can  stand  back,  lining  up  the  stop 
with  the  draw  tube,  and  see  whether  the  small  mirror  looks 
perfectly  round  and  is  concentric  with  the  reflected  circles.  If 
not,  a  touch  of  the  adjusting  screws  will  be  needed. 

Then  with  a  fine  pointed  brush  dot  the  center  of  the  mirror 
itself  through  the  hole,  with  white  paint.  Then,  removing  the 
card,  one  will  see  this  dot  accurately  centered  in  the  small  mirror 
if  the  large  one  is  in  adjustment,  and  it  remains  as  a  permanent 
reference  point.  If  the  dot  be  eccentric  it  can  be  treated  as 
before,  but  by  the  adjusting  screws  of  the  large  mirror. 

The  final  adjustment  can  then  be  made  by  getting  a  slightly 
extra-focal  star  image  fairly  in  the  center  of  the  field  with  a 
rather  high  power  and  making  the  system  concentric  as  before 
described.  This  sounds  a  bit  complicated  but  it  really  is  not. 
If  the  large  mirror  is  not  in  place,  its  counter  cell  may  well  be 
centered  and  levelled  by  help  of  a  plumb  line  from  the  center  of 
the  small  mirror  and  a  steel  square,  as  a  starting  point,  the  small 
mirror  having  been  centered  as  nearly  as  may  be  by  measure- 
ment.^ 

So  much  for  the  general  adjustment  of  the  objective  or  mirror. 
Its  actual  quality  is  shown  only  on  careful  examination. 

As  a  starting  point  one  may  take  the  extra-focal  system  of  rings 
given  by  an  objective  or  mirror  after  proper  centering.  If  the 
spherical  aberration  has  thoroughly  removed  the  appearance  of 
the  rings  when  expanded  so  that  six  or  eight  are  visible  should  be 
like  Fig.  158.  The  center  should  be  a  sharply  defined  bright 
point  and  surrounding  it,  and  exactly  concentric,  should  be  the 
^  Sometimes  with  ever  so  careful  centering  the  ring  system  in  the  middle 
of  the  field  is  still  eccentric  with  respect  to  the  small  mirror,  showing  that 
the  axis  of  the  parabola  is  not  perpendicular  to  the  general  face  of  the  mirror. 
This  can  usually  be  remedied  by  the  adjusting  screws  of  the  main  mirror  as 
described,  but  now  and  then  it  is  necessary  actually  to  move  over  the  small 
mirror  into  the  real  optical  axis.  Draper  (loc.  cit.)  gives  some  experiences 
of  this  sort. 


208 


THE  TELESCOPE 


Fig.  158. — Correct  Extra-focal  Image- 


interference  rings,  truly  circular  and  gradually  increasing  in 
intensity  outwards,  the  last  being  very  noticeably  the  strongest. 
One  can  best  make  the  test  when  looking  through  a  yellow 
glass  screen  which  removes  the  somewhat  confusing  flare  due  to 
imperfect  achromatism  and  makes  the  appearances  inside  and 

outside  focus  closely  similar. 
Just  inside  or  outside  of  focus 
the  appearance  should  be  that 
of  Fig.  159  for  a  perfectly  cor- 
rected objective  or  mirror. 

Sometimes  an  objective  will  be 
found  in  which  one  edge  of  the 
focussed  star  image  is  notably 
red  and  the  opposite  one  tinted 
with  greenish  or  bluish,  showing  unsymmetrical  coloring,  still 
more  obvious  when  the  image  is  put  a  little  out  of  focus.  This 
means  that  the  optical  centers  of  crown  and  flint  are  out  of  line 
from  careless  edging  of  the  lenses  or  very  bad  fitting.  The  case 
is  bad  enough  to  justify  trying  the  only  remedy  available  outside 
the  optician's  workshop — rotating  one  lens  upon  the  other  and  thus 
trying  the  pair  in  different  relative  azimuths. 

The  initial  positions  of  the  pair  must  be  marked  plainly,  care 
must  be  taken  not  to  displace  the  spacers  120°  apart  often  found 
at  the  edges  of  the  lenses,  and  the 
various  positions  must  be  tried  in 
an  orderly  manner.  One  not  in- 
frequently finds  a  position  in  which 
the  fault  is  negligible  or  disappears 
altogether,  which  point  should  be 
at  once  marked  for  reference. 

In  case  there  is  uncorrected 
spherical  aberration  there  is  de- 
parture from  regular  gradation  of  brightness  in  the  rings.  If 
there  is  a  "short  edge,"  i.e.,  +  spherical  aberration,  so  that  rays 
from  the  outer  zone  come  to  a  focus  too  short,  the  edge  ring  will 
look  too  strong  within  focus,  and  the  inner  rings  relatively  weak; 
with  this  appearance  reversed  outside  focus.  A  "long  edge"  i.e., 
—  spherical  aberration,  shows  the  opposite  condition,  edge  rings 
too  strong  outside  focus  and  too  weak  within.  Both  are  rather 
common  faults.  The  "long  edge"  effect  is  shown  in  Figs.  160 
and  161,  as  taken  quite  close  to  focus. 


Fig.  159. — Correct  Image  Just 
Out  of  Focus. 


THE  CARE  AND  TESTING  OF  TELESCOPES  209 

It  takes  a  rather  sharp  eye  and  considerable  experience  to 
detect  small  amounts  of  spherical  aberration;  perhaps  the  best 
way  of  judging  is  in  quickly  passing  from  just  inside  to  just  out- 
side focus  and  back  again,  using  a  yellow  screen  and  watching 
very  closely  for  variations  in  brightness.  Truth  to  tell  a  small 
amount  of  residual  aberration,  like  that  of  Fig.  160,  is  not  a 
serious  matter  as  regards  actual  performance — it  hurts  the 
telescopist's  feelings  much  more  than  the  quality  of  his  images. 

A  much  graver  fault  is  zonal  aberration,  where  some  inter- 
mediate zone  of  objective  or  mirror  comes  to  a  focus  too  long  or 
too  short,  generally  damaging  the  definition  rather  seriously, 


Fig.   160.  Fig.  161. 

Fig.  160. — Spherical  Aberration  Just  Inside  Focus. 
Fig.   161. — Spherical  Aberration  Just  Outside  Focus. 

depending  on  the  amount  of  variation  in  focus  of  the  faulty  zone. 
A  typical  case  is  shown  in  Fig.  162  taken  within  focus.  Here 
two  zones  are  abnormally  strong  showing,  just  as  in  the  case  of 
simple  spherical  aberration,  too  short  focus.  Outside  of  focus 
the  intensities  would  change  places,  the  outer  and  midway  zones 
and  center  being  heavy,  and  the  strong  zones  of  Fig.  162  weak. 
These  zonal  aberrations  are  easily  detected  and  are  rather  com- 
mon both  in  objectives  and  mirrors,  though  rarely  as  conspicuous 
as  in  Fig.  162. 

Another  failing  is  the  appearance  of  astigmatism,  which, 
broadly,  is  due  to  a  refracting  or  reflecting  surface  which  is  not 
a  surface  of  revolution  and  therefore  behaves  differently  for 
rays  incident  in  different  planes  around  its  optical  axis.  In  its 
commonest  form  the  surface  reflects  or  refracts  more  strongly 
along  one  plane  than  along  another  at  right  angles  to  it.  Hence 
the  two  have  different  foci  and  there  is  no  point  focus  at  all, 
but  two  line  foci  at  right  angles.  Figs.  163  and  164  illustrate 
this  fault,  the  former  being  taken  inside  and  the  latter  outside 
focus,  under  fairly  high  power.  If  a  star  image  is  oval  and  the 
major  axis  of  this  oval  has  turned  through  90°  when  one  passes  to 
the  other  side  of  focus,  astigmatism  is  somewhere  present. 

14 


210 


THE  TELESCOPE 


As  more  than  half  of  humanity  is  astigmatic,  through  fault  of 
the  eye,  one  should  twist  the  axis  of  the  eyes  some  90°  around  the 
axis  of  the  telescope  and  look  again.  If  the  axis  of  the  oval  has 
turned  with  the  eyes  a  visit  to  the  oculist  is  in  order.  If  not,  it  is 
worth  while  rotating  the  ocular.  If  the  oval  does  not  turn  with  it 
that  particular  telescope  requires  reworking  before  it  can  be  of 
much  use. 

This  astigmatism  due  to  fault  of  figure  must  not  be  confused 

with  the  astigmatic  difference  of  the 
image  surfaces  referred  to  in  Chapter 
IV  which  is  zero  on  the  axis  and  not 
of  material  importance  in  ordinary 
telescopes.  Astigmatism  of  figure  on 
the  contrary  is  bad  everywhere  and 
always.  It  should  be  especially  looked 
out  for  in  reflecting  surfaces,  curved 
or  plane,  since  it  is  a  common  result 
of  flexure. 
Passing  on  now  from  these  simple  tests  for  figure,  chromatic 
aberration  has  to  be  examined.  Nothing  is  better  than  an 
artificial  star  formed  by  the  sun  in  daylight,  for  the  preliminary 
investigation.  At  night  Polaris  is  advantageous  for  this  as  for 
other  tests. 


Fig.  162.— a  Case  of  Zonal 
Aberration. 


Fig.    1G3.  Fig.    164. 

Fig.  163. — Astigmatism  Inside  Focus. 
Fig.  164. — Astigmatism  Outside  Focus. 


The  achromatization  curves.  Fig.  63,  really  tell  the  whole  story 
of  what  is  to  be  seen.  When  the  telescope  is  carefully  focussed 
for  the  bright  part  of  the  spectrum,  getting  the  sharpest  star 
image  attainable,  the  central  disc,  small  and  clean,  should  be 
yellowish  white,  seen  under  a  power  of  60  or  70  per  inch  of 
aperture. 

But  the  red  and  blue  rays  have  a  longer  focus  and  hence  rim 
the  image  with  a  narrow  purplish  circle  varying  slightly  in  hue 
according  to  the  character  of  the  achromatization.     Pushing 


THE  CARE  AND  TESTING  OF  TELESCOPES  211 

the  ocular  a  little  inside,  focus,  the  red  somewhat  over-balances 
the  blue  and  the  purple  shades  toward  the  red.  Pulling  out  the 
ocular  very  slightly  one  brings  the  deep  red  into  focus  as  a  minute 
central  red  point,  just  as  the  image  begins  to  expand  a  little. 
Further  outside  focus  a  bluish  or  purplish  flare  fills  the  center  of 
the  field,  while  around  it  lies  a  greenish  circle  due  to  the  rays 
from  the  middle  of  the  secondary  spectrum  expanding  from 
their  shorter  focus. 

In  an  under-corrected  objective  this  red  point  is  brighter  and 
the  fringe  about  the  image,  focussed  or  within  focus,  is  con- 
spicuously reddish.  Heavy  over-correction  gives  a  strong  bluish 
fringe  and  the  red  point  is  dull  or  absent.  With  a  low  power  ocular, 
unless  it  be  given  a  color  correction  of  its  own,  any  properly  cor- 
rected objective  will  seem  under-corrected  as  already  explained. 

The  color  correction  can  also  be  well  examined  by  using  an 
ocular  spectroscope  like  Fig.  140,  with  the  cylindrical  lens 
removed.  Examining  the  focussed  star  image  thus,  the  spectrum 
is  a  narrow  line  for  the  middle  color  of  the  secondary  spectrum, 
widening  equally  at  F  and  B,  and  expanding  into  a  sort  of  brush 
at  the  violet  end.  Conversely,  when  moved  outside  focus  until 
the  width  is  reduced  to  a  narrow  line  at  F  and  B,  the  widening 
toward  the  yellow  and.  green  shows  very  clearly  the  nature  and 
extent  of  the  secondary  spectrum.  In  this  way  too,  the  actual 
foci  for  the  several  colors  can  easily  be  measured. 

The  exact  nature  of  the  color  correction  is  somewhat  a  matter 
of  taste  and  of  the  uses  for  which  the  telescope  is  designed,  but 
most  observers  agree  in  the  desirability  of  the  B-F  correction 
commonly  used  as  best  balancing  the  errors  of  eye  and  ocular. 
With  reflectors,  achromatic  or  even  over-corrected  oculars  are 
desirable.  The  phenomena  in  testing  a  telescope  for  color 
vary  with  the  class  of  star  observed — the  solar  type  is  a  good 
average.  Trying  a  telescope  on  a  Lyrae  emphasizes  unduly  the 
blue  phases,  while  a  Orionis  would  overdo  the  red. 

The  simple  tests  on  star  discs  in  and  out  of  focus  here  described 
are  ample  for  all  ordinary  purposes,  and  a  glass  which  passes 
them  well  is  beyond  question  an  admirably  figured  one.  The 
tests  are  not  however  quantitative,  and  it  takes  an  experienced 
eye  to  pick  out  quickly  minor  errors,  which  are  somewhat  irregu- 
lar. One  sometimes  finds  the  ring  system  excellent  but  a  sort  of 
haze  in  the  field,  making  the  contrasts  poor — bad  polish  or  dirt, 
but  figure  good. 


212 


THE  TELESCOPE 


A  test  found  very  useful  by  constructors  or  those  with  labora- 
tory facilities  is  the  knife  edge  test,  worked  out  chiefly  by  Fou- 
cault  and  widely  used  in  examining  specula.  It  consists  in 
principle  of  setting  up  the  mirror  so  as  to  bring  the  rays  to  the 
sharpest  possible  focus.  For  instance  in  a  spherical  mirror  a 
lamp  shining  through  a  pin  hole  is  placed  in  the  centre  of  curva- 
ture, and  the  reflected  image  is  brought  just  alongside  it  where  it 
can  be  inspected  by  eye  or  eyepiece.     In  Fig.  165  all  the  rays 


Fig.  165. — The  Principle  of  the  Foucault  Test. 

which  emanate  from  the  pinhole  h  and  fall  on  the  mirror  a  are 
brought  quite  exactly  to  focus  at  c.  The  eye  placed  close  to  c 
will,  see  if  the  mirror  surface  is  perfect,  a  uniform  disc  of  light 
from  the  mirror. 

If  now  a  knife  edge  like  d,  say  a  safety  razor  blade,  be  very 
gradually  pushed  through  the  focus  the  light  will  be  cut  off  in 


Taraboloidal 


I- 


Mirror  I  I  Mirror 

1( 
Focus?/ 

Fig.  166. — Foucault  Test  of  Parabolic  Mirror. 

a  perfectly  uniform  manner — no  zone  or  local  spot  going  first. 
If  some  error  in  the  surface  at  any  point  causes  the  reflected  ray 
to  miss  the  focus  and  cross  ahead  of  or  behind  it  as  in  the  ray 
hef,  then  the  knife  edge  will  catch  it  first  or  last  as  the  case  may 
be,  and  the  spot  e  will  be  first  darkened  or  remain  bright  after 
the  light  elsewhere  is  extinguished. 

One  may  thus  explore  the  surface  piecemeal  and  detect  not 
only  zones  but  slight  variations  in  the  same  zone  with  great 
precision.     In  case  of  a  parabolic  mirror  as  in  Fig.  166  the  test 


THE  CARE  AND  TESTING  OF  TELESCOPES  213 

is  made  at  the  focus  by  aid  of  the  auxihary  plane  mirror,  and  a 
diagonal  as  shown,  the  pinhole  and  knife  edge  being  arranged 
quite  as  before.  A  very  good  description  of  the  practical  use  of 
the  knife  edge  test  may  be  found  in  the  papers  of  Dr.  Draper  and 
Mr.  Ritchey  already  cited. 

It  is  also  applied  to  refractors,  in  which  case  monochromatic 
light  had  better  be  used,  and  enables  the  experimenter  to  detect 
even  the  almost  infinitesimal  markings  sometimes  left  by  the 
polishing  tool,  to  say  nothing  of  slight  variations  in  local  figure 
which  are  continually  lost  in  the  general  illumination  about  the 
field  when  one  uses  the  star  test  in  the  ordinary  manner. 

The  set-up  for  the  knife  edge  experiments  should  be  very  steady 
and  smooth  working  to  secure  precise  results,  and  it  therefore  is 
not  generally  used  save  in  the  technique  of  figuring  mirrors,  where 
it  is  invaluable.  With  micrometer  motions  on  the  knife  edge, 
crosswise  and  longitudinally,  one  can  make  a  very  exact  diagnosis 
of  errors  of  figure  or  flexure. 

A  still  more  delicate  method  of  examining  the  perfection  of 
figuring  is  found  in  the  Hartmann  test.  (Zeit.  fur  Instk., 
1904,  1909).  This  is  essentially  a  photographic  test,  comparing 
the  effect  of  the  individual  zones  of  the  objective  inside  and  out- 
side of  focus.  Not  only  are  the  effects  of  the  zones  compared  but 
also  the  effects  of  different  parts  of  the  same  zone,  so  that  any  lack 
of  symmetry  in  performance  can  be  at  once  found  and  measured. 

The  Hartmann  test  is  shown  diagrammatically  in  Fig.  167. 
The  objective  is  set  up  for  observing  a  natural  or  artificial  star. 
Just  in  front  of  it  is  placed  an  opaque  screen  perforated  with 
holes,  as  shown  in  section  by  Fig.  167,  where  A  is  the  perforated 
screen.  The  diameters  of  the  holes  are  about  J^^o  the  diameter  of 
the  objective  as  the  test  is  generally  applied,  and  there  are 
usually  four  holes  90°  apart  for  each  zone.  And  such  holes  are 
not  all  in  one  line,  but  are  distributed  symmetrically  about  the 
screen,  care  being  taken  that  each  zone  shall  be  represented  by 
holes  separated  radially  and  also  tangentially,  corresponding  to 
the  pairs  of  elements  in  the  two  astigmatic  image  surfaces,  an 
arrangement  which  enables  the  astigmatism  as  well  as  figure  to 
be  investigated. 

The  arrangement  of  holes  actually  found  useful  is  shown  in 
Hartmann's  original  papers,  and  also  in  a  very  important  paper  by 
Plaskett  (Ap.  J.  25  195)  which  contains  the  best  account  in 
English  of  Hartmann's  methods  and  their  application.     Now 


214 


THE  TELESCOPE 


^-;r 


A  I 


THE  CARE  AND  TESTING  OF  TELESCOPES  215 

each  hole  in  the  screen  transmits  a  pencil  of  light  through  the 
objective  at  the  corresponding  point,  and  each  pencil  comes  to  a 
focus  and  then  diverges,  the  foci  being  distributed  somewhere  in 
the  vicinity  of  what  one  may  regard  as  the  principal  focus,  B. 
For  instance  in  Fig.  167  are  shown  five  pairs  of  apertures  a,  a', 
h,  h',  etc.,  in  five  different  zones.  Now  if  a  photographic  plate 
be  exposed  a  few  inches  inside  focus  as  at  C  each  pencil  from  an 
aperture  in  the  screen  will  be  represented  by  a  dot  on  the  photo- 
graph, at  such  distance  from  the  axis  and  from  the  corresponding 
dot  on  the  other  side  of  the  axis  as  the  respective  inclinations  of 
the  pencils  of  light  may  determine. 

Similarly  a  plate  exposed  at  approximately  equal  distance  on 
the  other  side  of  the  general  focus,  as  at  D,  will  show  a  pattern  of 
dots  due  to  the  distribution  of  the  several  rays  at  a  point  beyond 
focus.  Now  if  all  the  pencils  from  the  several  apertures  met  at 
a  common  focus  in  B,  the  two  patterns  on  the  plates  C  and  D 
would  be  exactly  alike  and  for  equal  distance  away  from  focus  of 
exactly  the  same  size.  In  general  the  patterns  will  not  exactly 
correspond,  and  the  differences  measured  with  the  micrometer 
show  just  how  much  any  ray  in  question  has  departed  from 
meeting  at  an  exact  common  focus  with  its  fellows. 

For  instance  in  the  cut  it  will  be  observed  that  the  rays  e  and 
^  focus  barely  beyond  C  arid  by  the  time  they  reach  D  are  well 
spread  apart.  The  relative  distance  of  the  dots  upon  these 
corresponding  plates,  with  the  distance  between  the  plates,  shows 
exactly  at  what  point  between  C  and  D  these  particular  rays 
actually  did  cross  and  come  to  a  focus. 

Determining  this  is  merely  a  matter  of  measuring  up  similar 
triangles,  for  the  path  of  the  rays  is  straight.  Similarly  inspec- 
tion will  show  that  the  rays  d  and  d'  meet  a  little  short  of  B,  and 
measurement  of  their  respective  records  on  the  plates  C  and  D 
would  show  the  existence  of  a  zone  intermediate  in  focus  between 
the  focus  of  e,e'  and  the  general  focus  at  B.  The  exact  departure 
of  this  zone  from  correct  focus  can  therefore  be  at  once  measured. 

A  little  further  examination  discloses  the  fact  that  the  outer 
zone  represented  by  the  rays  a, 6,  and  a'h'  has  not  quite  the  same 
focus  at  the  two  extremities  of  the  same  diameter  of  the  objective. 
In  other  words  the  lens  is  a  little  bit  flatter  at  one  end  of  this 
diameter  than  it  is  at  the  other,  so  that  the  rays  here  have  con- 
siderably longer  focus  than  they  should,  a  fault  by  no  means 
unknown  although  fortunately  not  very  common. 


216  THE  TELESCOPE 

It  will  be  seen  that  the  variations  between  the  two  screen 
patterns  on  C  and  D,  together  with  the  difference  between  them, 
give  accurately  the  performance  of  each  point  of  the  objective 
represented  by  an  aperture  in  the  screen.  And  similar  investiga- 
tions by  substantially  the  same  method  may  be  extended  to  the 
astigmatic  variations,  to  the  general  color  correction,  and  to  the 
difference  in  the  aberrations  for  the  several  colors.  The  original 
papers  cited  should  be  consulted  for  the  details  of  applying  this 
very  precise  and  interesting  test. 

It  gives  an  invaluable  record  of  the  detailed  corrections  of  an 
objective,  and  while  it  is  one  with  which  the  ordinary  observer 
has  little  concern  there  are  times  when  nothing  else  can  give  with 
equal  precision  the  necessary  record  of  performance.  There 
are  divers  other  tests  used  for  one  purpose  or  another  in  examin- 
ing objectives  and  mirrors,  but  those  here  described  are  ample 
for  nearly  all  practical  purposes,  and  indeed  the  first  two  com- 
monly disclose  all  that  it  is  necessary  to  know. 

Now  and  then  one  has  to  deal  with  an  objective  which  is 
unmitigatedly  dirty.  It  can  be  given  a  casual  preliminary  clean- 
ing in  the  way  already  mentioned,  but  sometimes  even  this  will 
not  leave  it  in  condition  for  testing.  Then  one  must  get  down 
to  the  bottom  of  things  and  make  a  thorough  job  of  it. 

The  chief  point  to  remember  in  undertaking  this  is  that  the 
thing  which  one  is  cleaning  is  glass,  and  very  easy  to  scratch  if 
one  rubs  dust  into  it,  but  quite  easy  to  clean  if  one  is  careful. 
The  second  thing  to  be  remembered  is  that  once  cleaned  it  must 
be  replaced  as  it  was  before  and  not  in  some  other  manner. 

The  possessor  of  a  dirty  objective  is  generally  advised  to  take 
it  to  the  maker  or  some  reliable  optician.  If  the  maker  is  handy, 
or  an  optician  of  large  experience  in  dealing  with  telescope 
objectives  is  available,  the  advice  is  good,  but  there  is  no  dijEficulty 
whatever  in  cleaning  an  objective  with  the  exercise  of  that 
ordinary  care  which  the  user  of  a  telescope  may  be  reasonably 
expected  to  possess. 

It  is  a  fussy  job,  but  not  difficult,  and  the  best  advice  as  to  how 
to  clean  a  telescope  objective  is  to  "tub"  it,  literally,  if  beyond 
the  stage  where  the  superficial  wiping  described  is  sufficient. 

To  go  about  the  task  one  first  sets  down  the  objective  in  its 
cell  on  a  horizontal  surface  and  removes  the  screws  that  hold  in 
the  retaining  ring,  or  unscrews  the  ring  itself  as  the  case  may  be. 
This  leaves  the  cell  and  objective  with  the  latter  uppermost  and 


THE  CARE  AND  TESTING  OF  TELESCOPES  217 

free  to  be  taken  out.  Prepare  on  a  table  a  pad  of  anything  soft, 
a  little  smaller  than  the  objective,  topping  the  pad  with  soft 
and  clean  old  cloth;  then,  raising  up  the  cell  at  an  edge,  slip  the 
two  thumbs  under  it  and  lay  the  fingers  lightly  on  the  outer  lens 
of  the  objective,  then  invert  the  whole  affair  upon  the  pad  and 
lift  off  the  cell,  leaving  the  objective  on  its  soft  bed. 

Before  anything  else  is  done  the  edge  of  the  objective  should 
be  marked  with  a  hard  lead  pencil  on  the  edge  of  both  the  com- 
ponent lenses,  making  two  well  defined  v's  with  their  points 
touching.  Also,  if,  as  usual,  there  are  three  small  separators 
between  the  edges  of  the  flint  and  crown  lenses,  mark  the  position 
of  each  of  these  1,  2,  3,  with  the  same  pencil. 

Forming  another  convenient  pad  of  something  soft,  lift  off  the 
upper  lens,  take  out  the  three  separators  and  lay  them  in  order 
on  a  sheet  of  paper  without  turning  them  upside  down.  Mark 
alongside  each,  the  serial  number  denoting  its  position.  Then 
when  these  spacers,  if  in  good  condition,  are  put  back,  they  will 
go  back  in  the  same  place  rightside  up,  and  the  objective  itself 
will  go  back  into  place  unchanged. 

Now  have  at  hand  a  wooden  or  fibre  tub  or  basin  which  has 
been  thoroughly  washed  out  with  soap  and  water  and  wiped 
dry.  Half  fill  it  with  water  slightly  lukewarm  and  with  a  good 
mild  toilet  soap,  shaving  soap  for  example,  with  clean  hands  and 
very  soft  clean  cloth,  go  at  one  of  the  lenses  and  give  it  a  thorough 
washing.  After  this  it  should  be  rinsed  very  thoroughly  and 
wiped  dry.  As  to  material  for  wiping,  the  main  thing  is  that  it 
must  be  soft  and  free  from  dust  that  will  scratch.  Old  handker- 
chiefs serve  a  good  turn. 

Dr.  Brashear  years  ago  in  describing  this  process  recommended 
cheese  cloth.  The  present  day  material  that  goes  under  this 
name  is  far  from  being  as  soft  at  the  start  as  it  ought  to  be,  and 
only  the  best  quality  of  it  should  be  used,  and  then  only  after 
very  thorough  soaking,  rinsing  and  drying.  The  very  soft  towels 
used  for  cleaning  cut  glass,  if  washed  thoroughly  clean  and  kept 
free  from  dust,  answer  perfectly  well.  The  cheese  cloth  has  the 
advantage  of  being  comparatively  cheap  so  that  it  can  be  thrown 
away  after  use.  Whatever  the  cloth,  it  should  be  kept,  after 
thorough  washing  and  drying,  in  a  closed  jar. 

Rinsing  the  lens  thoroughly  and  wiping  it  clean  and  dry  is  the 
main  second  stage  of  cleansing.  It  sometimes  will  be  found  to  be 
badly  spotted  in  a  way  which  this  washing  will  not  remove. 


218  THE  TELESCOPE 

Sometimes  the  spotting  will  yield  to  alcohol  carefully  rubbed 
on  with  soft  absorbent  cotton  or  a  bunch  of  lens  paper. 

If  alcohol  fails  the  condition  of  the  surface  is  such  as  to  justify 
trying  more  strenuous  means.  Nitric  acid  of  moderate  strength 
rubbed  on  with  a  swab  of  absorbent  cotton  will  sometimes  clear 
up  the  spotting.  If  this  treatment  be  used  it  should  be  followed 
up  with  a  10  per  cent  solution  of  pure  caustic  potash  or  moder- 
ately strong  ft.p.  ammonia  and  then  by  very  thorough  rinsing. 
Glass  will  stand  without  risk  cautious  application  of  both  acid 
and  alkali,  but  the  former  better  than  the  latter. 

Then  a  final  rinsing  and  drying  is  in  order.  Many  operators 
use  a  final  washing  with  alcohol  of  at  least  90  per  cent  strength 
which  is  allowed  to  evaporate  with  little  or  no  wiping.  Alcohol 
denatured  with  methyl  alcohol  serves  well  if  strong  enough  but 
beware  denatured  alcohol  of  unknown  composition.  Others 
have  used  petroleum  naphtha  and  things  of  that  sort.  At  the 
present  time  these  commercial  petroleum  products  are  extremely 
uncertain  in  quality,  like  gasoline,  being  obtained,  Heaven  knows 
how,  from  the  breaking  down  of  heavier  petroleum  products. 

If  pure  petroleum  ether  can  be  obtained  it  answers  quite  as 
well  as  alcohol,  but  unless  the  volatile  fluid  is  pure  it  may  leave 
streaks.  Ordinarily  neither  has  to  be  used,  as  after  the  proper 
wiping  the  glass  comes  perfectly  clean.  This  done  the  glass  can 
be  replaced  on  the  pad  whence  it  came  and  its  mate  put  through 
the  same  process. 

Flint  glass  is  more  liable  to  spot  than  the  crown,  but  the  crown 
is  by  no  means  immune  against  the  deterioration  of  the  surface, 
perhaps  incipient  devitrification,  and  during  the  war  many 
objectives  "went  blind"  from  unexplained  action  of  this  charac- 
ter. As  a  rule  the  soap  and  water  treatment  applied  with  care 
leaves  even  a  pretty  hard  looking  specimen  of  objective  in 
fairly  good  condition  except  for  the  scratches  which  previous 
users  have  put  upon  it. 

Then  if  the  spacing  pieces,  usually  of  tinfoil,  are  not  torn  or 
corroded  they  can  be  put  back  into  place,  the  one  lens  super- 
imposed upon  the  other,  and  the  pair  put  back  into  the  cell  by 
dropping  it  gently  over  them  and  re-inverting  the  whole,  taking 
care  this  time  to  have  soft  cloth  or  lens  paper  under  the  fingers. 
Then  the  retaining  ring  can  be  put  into  place  again  and  the 
objective  is  ready  for  testing  or  use  as  the  case  may  be. 

If  the  spacers  are  corroded  or  damaged  it  may  be  necessary 


THE  CARE  AND  TESTING  OF  TELESCOPES  219 

to  replace  them  with  very  thin  tinfoil  cut  the  same  size  and  shape, 
leaving  however  a  little  extra  length  to  turn  down  over  the  edge 
of  the  lower  lens.  They  are  fastened  in  place  on  the  extreme 
edge  only  by  the  merest  touch  of  mucilage,  shellac  or  Canada  bal- 
sam, whichever  comes  to  hand.  The  one  important  thing  is  that 
the  spacers  should  be  entirely  free  of  the  sticky  material  where 
they  lap  over  the  edge  of  the  lens  to  perform  the  separation. 
This  lap  is  generally  not  over  }'{q  of  an  inch,  not  enough  to  show 
at  the  outside  of  the  objective  when  it  is  in  its  cell.  When  the 
upper  lens  is  lightly  pressed  down  into  place,  after  the  gum  or 
shellac  is  dry,  all  the  projecting  portion  can  be  trimmed  away 
with  a  sharp  pen-knife  leaving  simply  the  spacers  in  the  appointed 
places  from  which  the  original  ones  were  removed. 

Some  little  space  has  been  given  to  this  matter  of  cleaning 
objectives,  as  in  many  situations  objectives  accumulate  dirt 
rather  rapidly  and  it  is  highly  desirable  for  the  user  to  learn  how 
to  perform  the  simple  but  careful  task  of  cleansing  them. 

In  ordinary  use,  when  dirt  beyond  the  reach  of  mere  dusting 
with  a  camel's  hair  brush  has  stuck  itself  to  the  exterior  of  an 
objective,  a  succession  of  tufts  of  absorbent  cotton  or  wads  of 
lens  paper  at  first  dampened  with  pure  water  or  alcohol  and  then 
followed  lightly,  after  the  visible  dirt  has  been  gently  mopped  up, 
by  careful  wiping  with  the  same  materials,  will  keep  the  exterior 
surface  in  good  condition,  the  process  being  just  that  suggested 
in  the  beginning  of  this  chapter  as  the  ordinary  cleaning  up  pre- 
paratory to  a  thorough  examination. 

The  main  thing  to  be  avoided  in  the  care  of  a  telescope,  aside 
from  rough  usage  generally,  is  getting  the  objective  wet  and  then 
letting  it  take  its  chances  of  drying.  In  many  climates  dew  is  a 
very  serious  enemy  and  the  customary  dew  cap  three  or  four 
diameters  long,  bright  on  the  outside  and  blackened  within,  is  of 
very  great  service  in  lessening  the  deposit  of  dew  upon  the  glass. 
Also  the  dew  cap  keeps  out  much  stray  light  that  might  otherwise 
do  mischief  by  brightening  the  general  field.  In  fact  its  function 
as  a  light-trap  is  very  important  especially  if  it  is  materially 
larger  in  diameter  than  the  objective  and  provided  with  stops. 

The  finder  should  be  similarly  protected,  otherwise  it  will 
mysteriously  go  blind  in  the  middle  of  an  evening's  work  due 
to  a  heavy  deposit  of  moisture  on  the  objective.  The  effect 
is  an  onset  of  dimness  and  bad  definition  which  is  altogether 
obnoxious. 


220  THE  TELESCOPE 

As  regards  the  metal  parts  of  a  telescope  they  should  be  treated 
like  the  metal  parts  of  any  other  machine,  while  the  moving 
parts  require  from  time  to  time  a  little  touch  of  sperm  or  similar 
oil  like  every  other  surface  where  friction  may  occur. 

The  old  fashioned  highly  polished  and  lacquered  brass  tube 
was  practically  impossible  to  keep  looking  respectably  well 
provided  it  was  really  used  to  any  considerable  extent.  About 
the  most  that  could  be  done  to  it  was  dusting  when  dusty,  and 
cautiously  and  promptly  wiping  off  any  condensed  moisture. 
The  more  modern  lacquered  tubes  require  very  little  care  and  if 
they  get  in  really  bad  condition  can  be  relacquered  without  much 
expense  or  difficulty. 

Wooden  tubes,  occasionally  found  in  old  instruments,  demand 
the  treatment  which  is  accorded  to  other  highly  finished  wooden 
things,  occasional  rubbing  with  oil  or  furniture  polish  according 
to  the  character  of  the  original  surface.  Painted  tubes  may 
occasionally  require  a  fresh  coat,  which  it  does  not  require  great 
skill  to  administer.  If  the  surface  of  wooden  tripods  comes  to 
be  in  bad  shape  it  needs  the  oil  or  polish  which  would  be  accorded 
to  other  well  finished  wooden  articles. 

Mountings  are  usually  painted  or  lacquered  and  either  surface 
can  be  renewed  eventually  at  no  great  trouble.  Bright  parts 
may  be  lightly  touched  with  oil  as  an  ordinary  rust  preventive. 

Reflecting  telescopes  are  considerably  more  troublesome  to 
keep  in  order  than  refractors  owing  to  the  tender  nature  of  the 
silvered  surface.  It  may  remain  in  good  condition  with  fairly 
steady  use  for  several  years  or  it  may  go  bad  in  a  few  months  or  a 
few  weeks.  The  latter  is  not  an  unusual  figure  in  telescopes 
used  about  a  city  where  smoke  is  plentiful.  The  main  thing  is 
to  prevent  the  deposit  of  dew  on  the  mirror,  or  getting  it  wet  in 
any  other  way,  for  in  drying  off  the  drops  almost  invariably 
leave  spots. 

Many  schemes  have  been  proposed  for  the  prevention  of  injury 
to  the  mirror  surface.  A  close  fitting  metal  cover,  employed 
whenever  the  mirror  is  not  in  use,  has  given  good  results  in  many 
places.  Where  conditions  are  extreme  this  is  sometimes  lined 
with  a  layer  of  dry  absorbent  cotton  coming  fairly  down  upon 
the  mirror  surface,  and  if  this  muffler  is  dry,  clean,  and  a  little 
warmer  than  the  mirror  when  put  on,  it  seems  to  be  fairly 
effective.     Preferably  the  mirror  should  be  kept,  when  not  in 


THE  CARE  AND  TESTING  OF  TELESCOPES  221 

use,  at  a  little  higher  temperature  than  the  surrounding  air  so 
that  dew  will  not  tend  to  deposit  upon  it. 

As  to  actual  protective  measures  the  only  thing  that  seems 
to  be  really  efficient  is  a  very  thin  coating  of  lacquer,  first  tried  by 
Perot  at  the  Paris  Observatory.  The  author  some  ten  years 
since  took  up  the  problem  in  protecting  some  laboratory  mirrors 
against  fumes  and  moisture  and  found  that  the  highest  grade  of 
white  lacquer,  such  as  is  used  for  the  coating  of  fine  silverware  in 
the  trade,  answered  admirably  if  diluted  with  six  or  eight  volumes 
of  the  thinner  sold  with  such  commercial  lacquers.  It  is  best  to 
thin  the  lacquer  to  the  requisite  amount  and  then  filter. 

If  now  a  liberal  amount  of  the  mixture  is  poured  upon  the 
mirror  surface  after  careful  dusting,  swished  quickly  around, 
and  the  mirror  is  then  immediately  turned  up  on  edge  to  drain 
and  dry,  a  very  thin  layer  of  lacquer  will  be  left  upon  it,  only  a 
fraction  of  a  wave  length  thick,  so  that  it  shows  broad  areas  of 
interference  colors. 

Treated  in  this  way  and  kept  dry  the  coating  will  protect  the 
brilliancy  of  the  silver  for  a  good  many  months  even  under  rather 
unfavorable  circumstances.  After  trying  out  the  scheme  rather 
thoroughly  the  treatment  was  applied  to  the  24  inch  reflector  of 
the  Harvard  Observatory  and  has  been  in  use  ever  since.  The 
author  applied  the  first  coating  in  the  spring  of  1913,  and  since 
that  time  it  has  only  been  necessary  to  resilver  perhaps  once  in 
six  months  as  against  about  as  many  weeks  previously. 

The  lacquer  used  in  this  case  was  the  so-called  "Lastina" 
lacquer  made  by  the  Egyptian  Lacquer  Company  of  New  York, 
but  there  are  doubtless  others  of  similar  grade  in  the  market. 
It  is  a  collodion  lacquer  and  in  recent  years  it  has  proved  desirable 
to  use  as  a  thinner  straight  commercial  amylacetate  rather  than 
the  thinner  usually  provided  with  the  lacquer,  perhaps  owing  to 
the  fact  that  difficulty  of  obtaining  materials  during  the  war 
may  have  caused,  as  in  so  many  other  cases,  substitutions  which, 
while  perfectly  good  for  the  original  purpose  did  not  answer  so 
well  under  the  extreme  conditions  required  in  preserving  telescope 
mirrors. 

The  lacquer  coating  when  thinned  to  the  extent  here  recom- 
mended does  not  apparently  in  any  way  deteriorate  the  definition 
as  some  years  of  regular  work  at  Harvard  have  shown.  Some 
experimenters  have,  however,  found  difficulty,  quite  certainly 
owing  to  using  too  thick  a  lacquer.     The  endurance  of  a  lacquer 


222  THE  TELESCOPE 

coating  where  the  mirror  is  kept  free  from  moisture,  and 
its  power  to  hold  the  original  brilliancy  of  the  surface  is  very 
extraordinary. 

The  writer  took  out  and  tested  one  laboratory  mirror  coated 
seven  years  before,  and  kept  in  a  dry  place,  and  found  the 
reflecting  power  still  a  little  above  .70,  despite  the  fact  that  the 
coating  was  so  dry  as  to  be  almost  powdery  when  touched  with  a 
tuft  of  cotton.  At  the  start  the  mirror  had  seen  some  little  use 
unprotected  and  its  reflection  coefficient  was  probably  around  .80. 
If  the  silver  coating  is  thick  as  it  can  be  conveniently  made,  on  a 
well  coated  mirror,  the  coat  of  lacquer,  when  tarnish  has  begun, 
can  be  washed  off  with  amylacetate  and  tufts  of  cotton  until  the 
surface  is  practically  clear  of  it,  and  the  silver  itself  repolished 
by  the  ordinary  method  and  relacquered. 

There  are  many  silvering  processes  in  use  and  which  one 
should  be  chosen  for  resilvering  a  mirror,  big  or  little,  is  quite 
largely  a  matter  of  individual  taste,  and  more  particularly  experi- 
ence. The  two  most  used  in  this  country  are  those  of  Dr. 
Brashear  and  Mr.  Lundin,  head  of  the  Alvan  Clark  Corporation, 
and  both  have  been  thoroughly  tried  out  by  these  experienced 
makers  of  big  mirrors. 

The  two  processes  differ  in  several  important  particulars  but 
both  seem  to  work  very  successfully.  The  fundamental  thing 
in  using  either  of  them  is  that  the  glass  surface  to  be  silvered 
should  be  chemically  clean.  The  old  silver,  if  a  mirror  is  being 
resilvered,  is  removed  with  strong  nitric  acid  which  is  very 
thoroughly  rinsed  off  after  every  trace  of  silver  has  been  removed. 
Sometimes  a  second  treatment  with  nitric  acid  may  advantage- 
ously follow  the  first  with  more  rinsing.  The  acid  should  be 
followed  by  a  10  per  cent  solution  of  c.p.  caustic  potash  (some 
operators  use  c.p.  ammonia  as  easier  to  clear  away)  rinsed  off 
with  the  utmost  thoroughness. 

On  general  principles  the  last  rinsing  should  be  with  distilled 
water  and  the  glass  surface  should  not  be  allowed  to  dry  between 
this  rinsing  and  starting  the  silvering  process,  but  the  whole 
mirror  should  be  kept  under  water  until  the  time  for  silvering. 
In  Dr.  Brashear's  process  the  following  two  solutions  are  made 
up;  first  the  reducing  solution  as  follows: 

Rock  candy,  20  parts  by  weight. 

Strong  nitric  acid  (spec.  gr.  1.22),  1  part. 

Alcohol,  20  parts. 


THE  CARE  AND  TESTING  OF  TELESCOPES  223 

Distilled  water,  200  parts. 

This  improves  by  keeping  and  if  this  preparation  has  to  be 
hurried  the  acid,  sugar  and  distilled  water  should  be  boiled 
together  and  then  the  alcohol  added  after  the  solution  is  cooled. 

Second,  make  up  the  silvering  solution  in  three  distinct  por- 
tions; first  the  silver  solution  proper  as  follows: 

1.  2  parts  silver  nitrate.  20  parts  distilled  water. 
Second,  the  alkali  solution  as  follows: 

2.  13^^  parts  c.p.  caustic  potash.     20  parts  distilled  water. 
Third,  the  reserve  silver  solution  as  follows: 

3.  3^^  part  silver  nitrate.     16  parts  distilled  water. 

The  working  solution  of  silver  is  then  prepared  thus:  Gradu- 
ally add  to  the  silver  solution  No.  1  the  strongest  ammonia,  slowly 
and  with  constant  stirring.  At  first  the  solution  will  turn  dark 
brown  and  then  it  will  gradually  clear  up.  Ammonia  should  be 
added  only  just  to  the  point  necessary  to  clear  the  solution. 

Then  add  No.  2,  the  alkali  solution.  Again  the  mixture  will 
turn  dark  brown  and  must  be  cautiously  cleared  once  more  with 
ammonia  until  it  is  straw  colored  but  clear  of  precipitate. 
Finally  add  No.  3,  the  reserve  solution,  very  cautiously  with 
stirring  until  the  solution  grows  darker  and  begins  to  show  traces 
of  suspended  matter  which  will  not  stir  out.  Then  filter  the 
whole  through  absorbent  cotton  to  free  it  of  precipitate  and  it  is 
ready  for  use.     One  is  then  ready  for  the  actual  silvering. 

Now  there  are  two  ways  of  working  the  process,  with  the 
mirror  face  up,  or  face  down.  The  former  is  advantageous  in 
allowing  better  inspection  of  the  surface  as  it  forms,  and  also  it 
permits  the  mirror  of  a  telescope  to  be  silvered  without  removing 
it  from  the  cell,  as  was  in  fact  done  habitually  in  case  of  the  big 
reflector  of  the  Alleghany  Observatory  where  the  conditions 
were  such  as  to  demand  resilvering  once  a  month.  The  solution 
was  kept  in  motion  during  the  process  by  rocking  the  telescope 
as  a  whole. 

When  silvering  face  up  the  mirror  is  made  to  form  the  bottom 
of  the  silvering  vessel,  being  fitted  with  a  wrapping  of  strong 
paraffined  or  waxed  paper  or  cloth,  wound  several  times  around 
the  rim  of  the  mirror  and  carried  up  perhaps  half  the  thickness 
of  the  mirror  to  form  a  retainer  for  the  silvering  solution.  This 
band  is  firmly  tied  around  the  edge  of  the  mirror  making  a  water 
tight  joint.     Ritchey  uses  a  copper  band  fitted  to  the  edge  of 


224  THE  TELESCOPE 

the  mirror  and  drawn  tight  by  screws,  and  finishes  making 
tight  with  paraffin  and  a  warm  iron. 

In  silvering  face  down  the  mirror  is  suspended  a  Httle  distance 
above  the  bottom  of  a  shallow  dish,  preferably  of  earthen  ware, 
containing  the  solution.  Various  means  are  used  for  supporting 
it.  Thus  cleats  across  the  back  cemented  on  with  hard  optician's 
pitch  answer  well  for  small  mirrors,  and  sometimes  special 
provision  is  made  for  holding  the  mirror  by  the  extreme  edge  in 
clamps. 

Silvering  face  down  is  in  some  respects  less  convenient  but 
does  free  the  operator  from  the  very  serious  trouble  of  the  heavy 
sediment  which  is  deposited  from  the  rather  strong  silver  solution. 
This  is  the  essential  difficulty  of  the  Brashear  process  in  silvering 
face  up.  The  trouble  may  be  remedied  by  very  gentle  swabbing 
of  the  surface  under  the  liquid  with  absorbent  cotton,  from  the 
time  when  the  silver  coating  begins  fairly  to  form  until  it  is 
completed. 

The  Brashear  process  is  most  successfully  worked  at  a  tem- 
perature between  65°  and  70°  F.  and  some  experience  is  required 
to  determine  the  exact  proportion  of  the  reducing  solution  to  be 
added  to  the  silvering  solution.  Ritchey  advises  such  quantity 
of  the  reducing  solution  as  contains  of  sugar  one-half  the  total 
weight  of  the  silver  nitrate  used.  The  total  amount  of  solution 
after  mixing  should  cover  the  mirror  about  an  inch  deep.  Too 
much  increases  the  trouble  from  sediment  and  fails  to  give  a 
clean  coating.  The  requisite  quantity  of  reducing  solution  is 
poured  into  the  silvering  solution  and  then  immediately,  if  the 
mirror  is  face  up,  fairly  upon  it,  without  draining  it  of  the  water 
under  which  it  has  been  standing. 

If  silvering  face  down  the  face  will  have  been  immersed  in  a 
thin  layer  of  distilled  water  and  the  mixed  solutions  are  poured 
into  the  dish.  In  either  case  the  solution  is  rocked  and  kept 
moving  pretty  thoroughly  until  the  process  is  completed  which 
will  take  about  five  minutes.  If  silvering  is  continued  too  long 
there  is  likelihood  of  an  inferior  whitish  outer  surface  which  will 
not  polish  well,  but  short  of  this  point  the  thicker  the  coat  the 
better,  since  a  thick  coat  stands  reburnishing  where  a  thin  one 
does  not  and  moreover  the  thin  one  may  be  thin  enough  to 
transmit  some  valuable  light. 

When  the  silvering  is  done  the  solution  should  be  rapidly 
poured  off,  the  edging  removed  or  the  mirror  lifted  out  of  the 


THE  CARE  AND  TESTING  OF  TELESCOPES  225 

solution,  rinsed  off  first  with  tap  water  and  then  with  distilled, 
and  swabbed  gently  to  clear  the  remaining  sediment.  Then  the 
mirror  can  be  set  up  on  edge  to  dry.  A  final  flowing  with  alcohol 
and  the  use  of  a  fan  hastens  the  process. 

In  Lundin's  method  the  initial  cleaning  process  is  the  same  but 
after  the  nitric  acid  has  been  thoroughly  rinsed  off  the  surface  is 
gently  but  thoroughly  rubbed  with  a  saturated  solution  of  tin 
chloride,  applied  with  a  wad  of  absorbent  cotton.  After  the 
careful  rubbing  the  tin  chloride  solution  must  be  washed  off 
with  the  utmost  thoroughness,  preferably  with  moderately  warm 
water.  It  is  just  as  important  to  get  off  the  tin  chloride  com- 
pletely, as  it  is  to  clean  completely  the  mirror  surface  by  its  use. 
Otherwise  streaks  may  be  left  where  the  silvering  will  not  take 
well. 

When  the  job  has  been  properly  done  one  can  wet  the  whole 
surface  with  a  film  of  water  and  it  will  stay  wet  even  when  the 
surface  is  slightly  tilted.  As  in  the  Brashear  process  the  mirror 
must  be  kept  covered  with  water.  Mr.  Lundin  always  silvers 
large  mirrors  face  up,  and  forms  the  dish  by  wrapping  around  the 
edge  of  the  mirror  a  strip  of  bandage  cloth  soaked  in  melted 
beeswax  and  smoothed  off  by  pulling  it  while  still  hot  between 
metal  rods  to  secure  even  distribution  of  the  wax  so  as  to  make  a 
water  tight  joint.  This  rim  of  cloth  is  tied  firmly  around  the 
edge  of  the  mirror  and  the  strings  then  wet  to  draw  them 
still  tighter. 

Meanwhile  the  water  should  cover  the  mirror  by  ^  of  an  inch 
or  more.  It  is  to  be  noted  that  in  the  Lundin  process  ordinary 
water  is  usually  found  just  as  efficient  as  distilled  water,  but  it  is 
hardly  safe  to  assume  that  such  is  the  case,  without  trying  it  out 
on  a  sample  of  glass. 

There   are   then   prepared   two   solutions,    a   silver   solution, 

2.16  parts  silver  nitrate  (see  King,  Pop.  Ast  30,  93). 

100  parts  water, 
and  a  reducing  solution, 

4  parts  Merck's  formaldehyde 

20  parts  water. 
This  latter  quantity  is  used  for  each  100  parts  of  the  above  silver 
solution,  and  the  whole  quantity  made  up  is  determined  by  the 
amount  of  liquid  necessary  to  cover  the  mirror  as  just  described. 

The  silver  solution  is  cautiously  and  completely  cleared  up  by 
strong  ammonia  as  in  the  Brashear  process.     The  silver  and 

15 


226  THE  TELESCOPE 

reducing  solutions  are  then  mixed,  the  water  covering  the  mirror 
poured  quickly  off,  and  the  silvering  solution  immediately  poured 
on.  The  mirror  should  then  be  gently  rocked  and  the  silver  coating 
carefully  watched  as  it  forms. 

As  the  operation  is  completed  somewhat  coarse  black  grains 
of  sediment  will  form  and  when  these  begin  to  be  in  evidence  the 
solution  should  be  poured  off,  the  mirror  rinsed  in  running  water, 
the  edging  removed  while  the  mirror  is  still  rinsing  and  finally  the 
sediment  very  gently  swabbed  off  with  wet  absorbent  cotton. 
Then  the  mirror  can  be  set  up  to  dry. 

The  Lundin  process  uses  a  considerably  weaker  silver  solution 
than  the  Brashear  process,  is  a  good  deal  more  cleanly  while  in 
action,  and  is  by  experienced  workers  said  to  perform  best  at  a 
materially  lower  temperature  than  the  Brashear  process,  with  the 
mirror,  however,  always  slightly  warmer  than  the  solution. 
Some  workers  have  had  good  results  by  omitting  the  tin  chloride 
solution  and  cleaning  up  the  surface  by  the  more  ordinary 
methods.  In  the  Lundin  process  the  solution  is  sufficiently  clear 
for  the  density  acquired  by  the  silver  coating  to  be  roughly 
judged  by  holding  an  incandescent  lamp  under  the  mirror.  A 
good  coating  should  show  at  most  only  the  faintest  possible  out- 
line of  the  filament,  even  of  a  gas  filled  lamp. 

Whichever  process  of  silvering  is  employed,  and  both  work 
well,  the  final  burnishing  of  the  mirror  after  it  is  thoroughly 
dry  is  performed  in  the  same  way,  starting  by  tying  up  a  very 
soft  ball  of  absorbent  cotton  in  the  softest  of  chamois  skin. 

This  burnisher  is  used  at  first  without  any  addition,  simply  to 
smooth  and  condense  the  film  by  going  over  it  with  quick,  short, 
and  gentle  circular  strokes  until  the  entire  surface  has  been 
thoroughly  cleaned  and  begins  to  show  a  tendency  to  take  polish. 
Then  a  very  little  of  the  finest  optical  rouge  should  be  put  on  to 
the  same,  or  better  another,  rubber,  and  the  mirror  gone  steadily 
over  in  a  similar  way  until  it  comes  to  a  brilliant  polish. 

A  good  deal  of  care  should  be  taken  in  performing  this  operation 
to  avoid  the  settling  of  dust  upon  the  surface  since  scratches  will 
inevitably  result.  Great  pains  should  also  be  taken  not  to  take 
any  chance  of  breathing  on  the  mirror  or  in  any  other  way  getting 
the  surface  in  the  slightest  degree  damp.  Otherwise  it  will  not 
come  to  a  decent  polish. 

Numerous  other  directions  for  silvering  will  be  found  in  the 
literature,  and  all  of  them  have  been  successfully  worked  at  one 


THE  CARE  AND  TESTING  OF  TELESCOPES  227 

time  or  another.  The  fundamental  basis  of  the  whole  process  is 
less  in  the  particular  formula  used  than  in  the  most  scrupulous 
care  in  cleaning  the  mirror  and  keeping  it  clean  until  the  silvering 
is  completed.  Also  a  good  bit  of  experience  is  required  to  enable 
one  to  perform  the  operation  so  as  to  obtain  a  uniform  and  dense 
deposit. 


CHAPTER  X 
SETTING  UP  AND  HOUSING  THE  TELESCOPE 

In  regard  to  getting  a  telescope  into  action  and  giving  it 
suitable  protection,  two  entirely  different  situations  present 
themselves.  The  first  relates  to  portable  instruments  or  those 
on  temporary  mounts,  the  second  to  instruments  of  position. 
As  respects  the  two,  the  former  ordinarily  implies  general  use 
for  observational  purposes,  the  latter  at  least  the  possibility 
of  measurements  of  precision,  and  a  mount  usually  fitted  with 
circles  and  with  a  driving  clock.  Portable  telescopes  may  have 
either  altazimuth  or  equatorial  mounting,  while  those  permanent- 
ly set  up  are  now  quite  universally  equatorials. 

Portable  telescopes  are  commonly  small,  ranging  from  about 
23^^  inches  to  about  5  inches  in  aperture.  The  former  is  the  small- 
est that  can  fairly  be  considered  for  celestial  observations.  If 
thoroughly  good  and  well  mounted  even  this  is  capable  of  real 
usefulness,  while  the  five  inch  telescope  if  built  and  equipped 
in  the  usual  way,  is  quite  the  heaviest  that  can  be  rated  as 
portable,  and  deserves  a  fixed  mount. 

Setting  up  an  altazimuth  is  the  simplest  possible  matter. 
If  on  a  regular  tripod  it  is  merely  taken  out  and  the  tripod 
roughly  levelled  so  that  the  axis  in  azimuth  is  approximately 
vertical.  Now  and  then  one  sets  it  deliberately  askew  so  that 
it  may  be  possible  to  pass  quickly  between  two  objects  at  some- 
what different  altitudes  by  swinging  on  the  azimuth  axis. 

If  one  is  dealing  with  a  table  tripod  like  Fig.  69  it  should  merely 
be  set  on  any  level  and  solid  support  that  may  be  at  hand,  the 
main  thing  being  to  get  it  placed  so  that  one  may  look  through 
it  conveniently.  This  is  a  grave  problem  in  the  case  of  all 
small  refractors,  which  present  their  oculars  in  every  sort  of 
unreachable  and  uncomfortable  position. 

Of  course  a  diagonal  eyepiece  promises  a  way  out  of  the 
difficulty,  but  with  small  apertures  one  hesitates  to  lose  the 
light,  and  often  something  of  definition,  and  the  observer  must 
pretty  nearly  stand  on  his  head  to  use  the  finder.     With  well 

228 


SETTING  UP  AND  HOUSING  THE  TELESCOPE  229 

adjusted  circles,  such  are  commonly  found  on  a  fixed  mount, 
location  of  objects  is  easy.  On  a  portable  set-up  perhaps  the 
easiest  remedy  is  a  pair  of  well  aligned  coarse  sights  near  the 
objective  end  of  the  tube  and  therefore  within  reach  when 
it  is  pointed  zenith-ward.  The  writer  has  found  a  low,  arm- 
less, cheap  splint  rocker,  such  as  is  sold  for  piazza  use, 
invaluable  under  these  painful  circumstances,  and  can  cordially 
recommend  it. 

Even  better  is  an  observing  box  and  a  flat  cushion.  The 
box  is  merely  a  coverless  affair  of  any  smooth  %  inch  stuff 
firmly  nailed  or  screwed  together,  and  of  three  unequal  dimen- 
sions, giving  three  available  heights  on  which  to  sit  or  stand. 
The  dimensions  originally  suggested  by  Chambers  (Handbook 
of  Astronomy,  II,  215)  were  21  X  12  X  15  inches,  but  the  writer 
finds  18  X  10  X  14  inches  a  better  combination. 

The  fact  is  that  the  ordinary  stock  telescope  tripod  is  rather 
too  high  for  sitting,  and  too  low  for  standing,  comfortably. 
A  somewhat  stubby  tripod  is  advantageous  both  in  point  of 
steadiness  and  in  accessibility  of  the  eye-piece  when  one  is 
observing  within  30°  of  the  zenith,  where  the  seeing  is  at  its 
best;  and  a  sitting  position  gives  a  much  greater  range  of  con- 
venient upward  vision  than  a  standing  one. 

When  an  equatorial  mount  is  in  use  one  faces  the  question  of 
adjustment  in  its  broadest  aspect.  Again  two  totally  different 
situations  arise  in  using  the  telescope.  First  is  the  ordinary 
course  of  visual  observation  for  all  general  purposes,  in  which 
no  precise  measurements  of  position  or  dimensions  are  involved. 

Here  exact  following  is  not  necessary,  a  clock  drive  is  con- 
venient rather  than  at  all  indispensable,  and  even  circles  one 
may  get  along  without  at  the  cost  of  a  little  time.  Such  is  the 
usual  situation  with  portable  equatorials.  One  does  not  then 
need  to  adjust  them  to  the  pole  with  extreme  precision,  but 
merely  well  enough  to  insure  easy  following;  otherwise  one  is 
hardly  better  off  than  with  an  altazimuth. 

In  a  totally  different  class  falls  the  instrument  with  which  one 
undertakes  regular  micrometric  work,  or  enters  upon  an  extended 
spectroscopic  program  or  the  use  of  precise  photometric  appa- 
ratus, to  say  nothing  of  photography.  In  such  cases  a  permanent 
mount  is  almost  imperative,  the  adjustments  must  be  made 
with  all  the  exactitude  practicable,  one  finds  great  need  of 
circles,  and  the  lack  of  a  clock  drive  is  a  serious  handicap  or  worse. 


230  THE  TELESCOPE 

Moreover  in  this  latter  case  one  usually  has,  and  needs,  some 
sort  of  timepiece  regulated  to  sidereal  time,  without  which  a 
right  ascension  circle  is  of  very  little  use. 

In  broad  terms,  then,  one  has  to  deal,  first;  with  a  telescope 
on  a  portable  mount,  with  or  without  position  circles,  generally 
lacking  both  sidereal  clock  and  driving  clock,  and  located  where 
convenience  dictates;  second,  with  a  telescope  on  a  fixed  mount 
in  a  permanent  location,  commonly  with  circles  and  clock, 
and  with  some  sort  of  permanent  housing. 

Let  us  suppose  then  that  one  is  equipped  with  a  5  inch  instru- 
ment like  Fig.  168,  having  either  the  tripod  mount,  or  the  fixed 
pillar  mount  shown  alongside  it;  how  shall  it  be  set  up,  and,  if 
on  the  fixed  mount,  how  sheltered? 

In  getting  an  equatorial  into  action  the  fundamental  thing 
is  to  place  the  optical  axis  of  the  telescope  exactly  parallel  to 
the  polar  axis  of  the  mount  and  to  point  the  latter  as  nearly  as 
possible  at  the  celestial  pole. 

The  conventional  adjustments  of  an  equatorial  telescope  are 
as  follows: 

1.  Adjust  polar  axis  to  altitude  of  pole. 

2.  Adjust  index  of  declination  circle. 

3.  Adjust  polar  axis  to  the  meridian. 

4.  Adjust  optical  axis  perpendicular  to  declination  axis. 

5.  Adjust  declination  axis  perpendicular  to  polar  axis. 

6.  Adjust  index  of  right  ascension  circle,  and 

7.  Adjust  optical  axis  of  finder  parallel  to  that  of  telescope. 

Now  let  us  take  the  simplest  and  commonest  case,  the  adjust- 
ment of  a  portable  equatorial  on  a  tripod  mount,  when  the 
instrument  has  a  finder  but  neither  circles  nor  driving  clock. 
Adjustments  2  and  6  automatically  drop  out  of  sight,  5  vanishes 
for  lack  of  any  means  to  make  the  adjustment  ,  and  on  a  mount 
made  with  high  precision,  like  the  one  before  us,  4  is  negligible 
for  any  purpose  to  which  our  instrument  is  applicable. 

Adjustments  1,  3  and  7  are  left  and  these  should  be  performed 
in  the  order  7,  1,  3,  for  sake  of  simplicity.  To  begin  with  the 
finder  has  cross-wires  in  the  focus  of  its  eyepiece,  and  the  next 
step  is  to  provide  the  telescope  itself  with  similar  cross-wires. 

These  can  readily  be  made,  if  not  provided,  by  cutting  out  a 
disc  of  cardboard  to  fit  snugly  either  the  spring  collar  just  in  front 
of  a  positive  eye-piece  or  the  eyepiece  itself  at  the  diaphragm, 
if  an  ordinary  Huygenian.     Rule  two  diametral  lines  on  the 


SETTING  UP  AND  HOUSING  THE  TELESCOPE  231 

circle  struck  for  cutting  the  cardboard,  crossing  at  the  center, 
cut  out  the  central  aperture,  and  then  very  carefully  stretch 


Fig.   168. — Clark  5-inch  with  Tripod  and  Pier. 

over  it,  guided  by  the  diametral  lines,  two  very  fine  threads  or 
wires  made  fast  with  wax  or  shellac. 

Now  pointing  the  telescope  at  the  most  distant  well  defined 
object  in  view,  rotate  the  spring  collar  or  ocular,  when,  if  the 
crossing  of  the  threads  is  central,  their  intersection  should  stay 


232  THE  TELESCOPE 

on  the  object.  If  not  shift  a  thread  cautiously  until  the  error 
is  corrected. 

Keeping  the  intersection  set  on  the  object  by  clamping  the  tube, 
one  turns  attention  to  the  finder.  Either  the  whole  tube  is  adj  ust- 
able  in  its  supports  or  the  cross- wires  are  capable  of  adjustment 
by  screws  just  in  front  of  the  eyepiece.  In  either  case  finder  tube 
or  cross- wires  should  be  shifted  until  the  latter  bear  squarely  upon 
the  object  which  is  in  line  with  the  cross  threads  of  the  main 
telescope.  Then  the  adjusting  screws  should  be  tightened,  and 
the  finder  is  in  correct  alignment. 

As  to  adjustments  1  and  3,  in  default  of  circles  the  ordinary 
astronomical  methods  are  not  available,  but  a  pretty  close 
approximation  can  be  made  by  levelling.  A  good  machinist's 
level  is  quite  sensitive  and  reliable.  The  writer  has  one  picked 
out  of  stock  at  a  hardware  shop  that  is  plainly  sensitive  to  2' 
of  arc,  although  the  whole  affair  is  but  four  inches  long. 

Most  mounts  like  the  one  of  Fig.  168  have  a  mark  ruled  on 
the  support  of  the  polar  axis  and  a  latitude  scale  on  one  of  the 
cheek  pieces.  Adjustment  of  the  polar  axis  to  the  correct 
altitude  is  then  made  by  placing  the  level  on  the  declination 
axis,  or  any  other  convenient  place,  bringing  it  to  a  level,  and 
then  adjusting  the  tripod  until  the  equatorial  head  can  be  revolved 
without  disturbing  this  level.  Then  set  the  polar  axis  to  the 
correct  latitude  and  adjustment  number  1  is  complete  for  the 
purpose  in  hand. 

Lacking  a  latitude  scale,  it  is  good  judgment  to  mark  out 
the  latitude  by  the  help  of  the  level  and  a  paper  protractor.  To 
do  this  level  the  polar  axis  to  the  horizontal,  level  the  telescope 
tube  also,  and  clamp  it  in  declination  to  maintain  it  parallel. 
Then  fix  the  protractor  to  a  bit  of  wood  tied  or  screwed  to  the 
telescope  support,  drop  a  thin  thread  plumb  line  from  a  pin 
driven  into  the  wood,  the  declination  axis  being  still  clamped, 
note  the  protractor  reading,  and  then  raise  the  polar  axis  by 
the  amount  of  the  latitude. 

Next,  with  a  knife  blade  scratch  a  conspicuous  reference  line 
on  the  sleeve  of  the  polar  axis  and  its  support  so  that  when  the 
equatorial  head  is  again  levelled  carefully  you  can  set  approxi- 
mately to  the  latitude  at  once. 

Now  comes  adjustment  3,  the  alignment  of  the  polar  axis  to 
the  meridian.  One  can  get  it  approximately  by  setting  the 
telescope  tube  roughly  parallel  with  the  polar  axis  and,  sighting 


SETTING  UP  AND  HOUSING  THE  TELESCOPE 


233 


along  it,  shifting  the  equatorial  head  in  azimuth  until  the  tube 
points  to  the  pole  star.  Then  several  methods  of  bettering  the 
adjustment  are  available. 

At  the  present  date  Polaris  is  quite  nearly  1°  07'  from  the  true 
pole  and  describes  a  circle  of  that  radius  about  it  every  24  hours. 
To  get  the  correct  place  of  the  pole  with  reference  to  Polaris 
one  must  have  at  least  an  approximate  knowledge, of  the  place 
of  that  star  in  its  little  orbit,  technically  its  hour-angle.  With 
a  little  knowledge  of  the  stars  this  can  be  told  off  in  the  skies 
almost  as  easily  as  one  reckons  time  on  a  clock.     Fig.  169  is,  in 


Fig.  169.— The  Cosmic  Clock. 


fact,  the  face  of  the  cosmic  clock,  with  a  huge  sweeping  hour  hand 
that  he  who  runs  may  read. 

It  is  a  clock  in  some  respects  curious;  it  has  a  twenty-four 
hour  face  like  some  clocks  and  watches  designed  for  Continental 
railway  time;  the  hour  hand  revolves  backward,  ("counter- 
clockwise") and  it  stands  in  the  vertical  not  at  noon,  but  at  1.20 
Star  Time.  The  two  stars  which  mark  the  tip  and  the  reverse  end 
of  the  hour  hand  are  delta  Cassiopeee  and  zeta  Ursse  Majoris 
respectively.  The  first  is  the  star  that  marks  the  bend  in  the 
back  of  the  great  "chair,"  the  second  (Mizar),  the  star  which 
is  next  to  the  end  of  the  "dipper"  handle. 


234  THE  TELESCOPE 

One  or  the  other  is  above  the  horizon  anywhere  in  the  northern 
hemisphere.  Further,  the  Hne  joining  these  two  stars  passes 
almost  exactly  through  the  celestial  pole,  and  also  very  nearly 
through  Polaris,  which  lies  between  the  pole  and  5  Cassiopese. 
Consequently  if  you  want  to  know  the  hour-angle  of  Polaris 
just  glance  at  the  clock  and  note  where  on  the  face  5  Cassiopeae 
stands,  between  the  vertical  which  is  XXIV  o'clock,  and  the 
horizontal,  which  is  VI  (east)  or  XVIII  (west)  o'clock. 

You  can  readily  estimate  its  position  to  the  nearest  half 
hour,  and  knowing  that  the  great  hour  hand  is  vertical  (5  Cassi- 
opeae up)  at  I''  20™  or  (f  Ursse  Majoris  up)  at  XIIP  20"",  you  can 
make  a  fairly  close  estimate  of  the  sidereal  time. 

A  little  experience  enables  one  to  make  excellent  use  of  the 
clock  in  locating  celestial  objects,  and  knowledge  of  the  approxi- 
mate hour  angle  of  Polaris  thus  observed  can  be  turned  to  immedi- 
ate use  in  making  adjustment  3.  To  this  end  slip  into  the 
plane  of  the  finder  cross  wires  a  circular  stop  of  metal  or  paper 
having  a  radius  of  approximately  1°  15'  which  means  a  diameter 
of  0.52  inch  per  foot  of  focal  length. 

Then,  leaving  the  telescope  clamped  in  declination  as  it  was 
after  adjustment  1,  turn  it  in  azimuth  across  the  pole  until  the 
pole  star  enters  the  field  which,  if  the  finder  inverts  it  will  do  on 
the  other  side  of  the  center;  i.e.  if  it  stands  at  IV  to  the  naked 
eye  it  will  enter  the  field  apparently  from  the  XVI  o'clock  quarter. 
When  just  comfortably  inside  the  field,  the  axis  of  the  telescope 
is  pointing  substantially  at  the  pole. 

It  is  better  to  get  Polaris  in  view  before  slipping  in  the  stop 
and  if  it  is  clearly  coming  in  too  high  or  too  low  shift  the  altitude 
of  the  polar  axis  a  trifle  to  correct  the  error.  This  approximate 
setting  can  be  made  even  with  the  smallest  finder  and  on  any 
night  worth  an  attempt  at  observation. 

With  a  finder  of  an  inch  or  more  aperture  a  very  quick  and 
quite  accurate  setting  to  the  meridian  can  be  made  by  the  use 
of  Fig.  170,  which  is  a  chart  of  all  stars  of  8  mag.  or  brighter 
within  1°  30'  of  the  pole.  There  are  only  three  stars  besides 
Polaris  at  all  conspicuous  in  this  region,  one  quite  close  to 
Polaris,  the  other  two  forming  with  it  the  triangle  marked  on 
the  chart.  These  two  are,  to  the  left,  a  star  of  magnitude  6.4 
designated  B.  D.  88  112,  and  to  the  right  one  of  magnitude  7.0, 
B.  D.  89  13. 

The  position  of  the  pole  for  the  rest  of  the  century  is  marked 


SETTING  UP  AND  HOUSING  THE  TELESCOPE  235 

on  the  vertical  arrow  and  with  the  stars  in  the  field  of  the  finder 
one  can  set  the  cross  wires  on  the  pole,  the  instrument  remaining 
clamped  in  declination,  within  a  very  few  minutes  of  arc,  quite 
closely  enough  for  any  ordinary  use  of  a  portable  mount.  All 
this  could  be  done  even  better  with  the  telescope  itself,  but  it  is 
very  rare  to  find  an  eyepiece  with  sufficient  field. 

At  all  events  the  effect  of  any  error  likely  to  be  made  in  these 
adjustments  is  not  serious  for  the  purpose  in  hand,  since  if  one 


Fig.  170. — The  Pole  among  the  Stars. 

makes  an  error  of  a  minute  of  arc  in  the  setting  the  resulting 
displacement  of  a  star  in  the  field  will  even  in  the  most  unfavor- 
able case  reach  this  full  amount  only  after  6  hours  following. 
I.e.  with  any  given  eyepiece  an  error  of  adjustment  equal  to  the 
radius  of  the  field  will  still  permit  following  a  star  for  an  hour 
or  two  before  it  drifts  inconveniently  wide  of  the  center. 

Considerable  space  has  been  devoted  to  these  easy  approxi- 
mations in  setting  up,  since  the  directions  commonly  given  require 
circles  and  often  a  clock  drive. 

In  some  cases  one  has  to  set  up  a  portable  equatorial  where 
from  necessity  for  clear  sky  space,  Polaris  is  not  visible.  The 
best  plan  then  is  to  set  up  with  great  care  where  Polaris  can  be 
seen,  paying  especial  attention  to  the  levelling.  Then  establish 
two  meridian  marks  on  stakes  at  a  convenient  distance  by  turning 
the  telescope  180°  on  its  declination  axis  and  sighting  through  it 


236  THE  TELESCOPE 

in  both  directions.  Now  with  a  surveyor's  tape  transfer  the 
meridian  Hne  East  or  West  as  the  case  may  be  until  it  can  be 
used  where  there  is  clear  sky  room. 

Few  observers  near  a  city  can  get  good  sky  room,  from  the 
interference  of  houses,  trees  or  blazing  street  lamps,  and  the 
telescope  must  often  be  moved  from  one  site  to  another  to  reach 
different  fields.  In  such  case  it  is  wise  to  take  the  very  first 
step  toward  giving  the  telescope  a  local  habitation  by  establishing 
a  definite  placement  for  the  tripod. 

To  this  end  the  three  legs  should  be  firmly  linked  together  by 
chains  that  will  not  stretch — leg  directly  to  leg,  and  not  to  a 
common  junction.  Then  see  to  it  that  each  leg  has  a  strong 
and  moderately  sharp  metal  point,  and,  the  three  points  of 
support  being  thus  definitely  fixed,  establish  the  old  reliable 
point-slot-plane  bearing  as  follows: 

Lay  out  at  the  site  (or  sites)  giving  the  desired  clear  view,  a 
circle  scratched  on  the  ground  of  such  size  that  the  three  legs 
of  your  tripod  may  rest  approximately  on  its  periphery.  Then 
lay  out  on  the  circle  three  points  120°  apart.  At  each  point  sink 
a  short  post  12  to  18  inches  long  and  of  any  convenient  diameter, 
well  tarred,  and  firmly  set  with  the  top  levelled  off  quite  closely 
horizontal. 

To  the  top  of  each  bolt  a  square  or  round  of  brass  or  iron 
about  half  an  inch  thick.  The  whole  arrangement  is  indicated 
in  diagram  in  Fig.  171.  In  a  sink  a  conical  depression  such  as 
is  made  by  drilling  nearly  through  with  a  1  inch  twist  drill.  The 
angle  here  should  be  a  little  broader  than  the  point  on  the  tripod 
leg.  In  h  have  planed  a  V  shaped  groove  of  equally  broad  angle 
set  with  its  axis  pointing  to  the  conical  hole  in  a.  Leave  the 
surface  of  c  a  horizontal  plane. 

Now  if  you  set  a  tripod  leg  in  a,  another  in  the  slot  at  6  and 
the  third  on  c,  the  tripod  will  come  in  every  instance  to  the  same 
level  and  orientation.  So,  if  you  set  up  your  equatorial  carefully 
in  the  first  place  and  leave  the  head  clamped  in  azimuth,  you 
can  take  it  in  and  replace  it  at  any  time  still  in  adjustment  as 
exact  as  at  the  start.  And  if  it  is  necessary  to  shift  from  one 
location  to  another  you  can  do  it  without  delay  still  holding 
accurate  adjustment  of  the  polar  axis  to  the  pole,  and  avoiding 
the  need  of  readjustment. 

In  case'  the  instrument  has  a  declination  circle  the  original 
set-up  becomes  even  simpler.     One  has  only  to  level  the  tripod, 


SETTING  UP  AND  HOUSING  THE  TELESCOPE  237 

either  with  or  without  the  equatorial  head  in  place,  and  then  to 
set  the  polar  axis  either  vertical  or  horizontal,  levelling  the  tube 
with  it  either  by  placing  the  level  across  the  objective  cell 
perpendicular  to  the  declination  axis,  or  laying  it  along  the  tube 
when  horizontal. 

Then,  reading  the  declination  circle,  one    can  set  off  the  co- 
atitude  or  latitude  as  the  case  may  be  and,  leaving  the  telescope 


/ 


\ 


f 

\ 
\ 
\ 


(Z)' 

a®. 


/ 
/ 
/ 
/ 


Fig.   171. — A  Permanent  Foothold  for  the  Tripod. 

clamped  in  declination,  lower  or  raise  the  polar  axis  until  the  tube 
levels  to  the  horizontal.  When  the  mount  does  not  permit  wide 
adjustment  and  has  no  latitude  scale  one  is  driven  to  laying 
out  a  latitude  templet  and,  placing  a  straight  edge  under  the 
equatorial  head,  or  suspending  a  plumb  line  from  the  axis 
itself,  setting  it  mechanically  to  latitude. 

Now  suppose  we  are  dealing  with  the  same  instrument,  but 
are  planning  to  plant  it  permanently  in  position  on  its  pillar 
mount.  It  is  now  worth  while  to  make  the  adjustments  quite 
exactly,  and  to  spend  some  time  about  it.  The  pillar  is  commonly 
assembled  by  well  set  bolts  on  a  brick  or  concrete  pier.  The 
preliminary  steps  are  as  already  described. 

The  pillar  is  levelled  across  the  top,  the  equatorial  head,  which 
turns  upon  it  in  azimuth,  is  levelled  as  before,  the  adjustment 
being  made  by  metal  wedges  under  the  pillar  or  by  levelling 


238  THE  TELESCOPE 

screws  in  the  mount  If  there  are  any.  Then  the  latitude  is  set 
off  by  the  scale,  or  by  the  declination  circle,  and  the  polar  axis 
turned  to  the  approximate  meridian  as  already  described. 

There  is  likely  to  be  an  outstanding  error  of  a  few  minutes 
of  arc  which  should  in  a  permanent  mount  be  reduced  as  far  as 
practicable.  At  the  start  adjust  the  declination  of  the  optical 
axis  of  the  telescope  to  that  of  the  polar  axis.  This  is  done  in 
the  manner  suggested  by  Fig.  172. 

Here  p  is  the  polar  axis  and  d  the  declination  axis.  Now  if 
one  sights,  using  the  cross  wires,  through  the  telescope  a  star 


Fig.  172. — Aligning  the  Optical  Axis. 

near  the  meridian,  i.e.,  one  that  is  changing  in  declination  quite 
slowly,  starting  from  the  position  A  with  the  telescope  E.  of  the 
polar  axes,  and  turns  it  over  180°  into  the  position  B,  W.  of  the 
polar  axis,  the  prolongation  of  the  line  of  sight,  h,  will  fall  below 
a,  when  as  here  the  telescope  points  too  high  in  the  A  position. 

In  other  words  the  apparent  altitude  of  the  star  will  change 
by  twice  the  angle  between  A  and  p.  Read  both  altitudes  on 
the  declination  circle  and  split  the  difference  with  the  slow  motion 
as  precisely  as  the  graduation  of  the  declination  circle  permits. 

The  telescope  will  probably  not  now  point  exactly  at  the  star, 
but  as  the  tube  is  swung  from  the  A  to  the  B  position  and  back 
the  visible  stars  will  describe  arcs  of  circles  which  should  be 
nearly  concentric  with  the  field  as  defined  by  the  stop  in  the 
eyepiece.  If  not,  a  very  slight  touch  on  the  declination  slow 
motion  one  way  or  the  other  will  make  them  do  so  to  a  sufficient 
exactness,  especially  if  a  rather  high  power  eyepiece  is  used. 


SETTING  UP  AND  HOUSING  THE  TELESCOPE  239 

The  optical  axis  of  the  telescope  is  now  parallel  to  the  polar 
axis,  but  the  latter  may  be  slightly  out  of  position  in  spite  of 
the  preliminary  adjustment.  Now  reverting  to  the  polar  field 
of  Fig.  170,  swing  from  position  AtoB  and  back  again,  correcting 
any  remaining  eccentricity  of  the  star  arcs  around  the  pole  by 
cautious  shifting  of  the  polar  axis,  leaving  the  telescope  clamped 
in  declination.  The  first  centering  is  around  the  pole  of  the 
instrument,  the  second  around  the  celestial  pole  by  help  of  a 
half  dozen  small  stars  within  a  half  degree  on  both  sides  of  it, 
magnitudes  9  and  10,  easily  visible  in  a  3"  or  4"  telescope,  using 
the  larger  field  of  the  finder  for  the  coarse  adjustment. 

If  the  divided  circles  read  to  single  minutes  or  closer,  which 
they  generally  do  not  on  instruments  of  moderate  size,  one  can 
use  the  readings  to  set  the  polar  axis  and  the  declination  circle, 
and  to  make  the  other  adjustments  as  well. 

In  default  of  this  help,  the  declination  circle  adjustment  may 
be  set  to  read  90°  when  the  optical  axis  is  brought  parallel  to  the 
polar  axis,  and  after  the  adjustment  of  the  latter  is  complete, 
the  R.  A.  circle  can  be  set  by  swinging  up  the  telescope  in  the 
meridian  and  watching  for  the  transit  of  any  star  of  known  R.  A. 
over  the  central  cross  wire,  at  which  moment  the  circle  should 
be  clamped  to  the  R.  A.  thus  defined. 

Two  possible  adjustments  are  left,  the  perpendicularity  of  the 
polar  and  declination  axes,  and  that  of  the  optical  axis  to  the 
declination  axis.  As  a  rule  there  is  no  provision  for  either  of 
these,  which  are  supposed  to  have  been  carried  out  by  the  maker. 
The  latter  adjustment  if  of  any  moment  will  disclose  itself  as 
a  lateral  wobble  in  trying  to  complete  the  adjustment  of  optical 
axis  to  polar  axis.  It  can  be  remedied  by  a  liner  of  tinfoil  or 
even  paper  under  one  end  of  the  tube's  bearing  on  its  cradle. 
Adjustment  of  the  former  is  strictly  a  job  for  the  maker. 

For  details  of  the  rigorous  adjustments  on  the  larger  instru- 
ments the  reader  will  do  well  to  consult  Loomis'  Practical  Astron- 
omy page  28  and  following. ^  The  adjustments  here  considered 
are  those  which  can  be  effectively  made  without  driving  clock, 
finely  divided  circles,  or  exact  knowledge  of  sidereal  time.  The 
first  and  last  of  these  auxiliaries,  however,  properly  belong  with 
an  instrument  as  large  as  Fig.  168,  on  a  fixed  mount. 

There  are  several  rather  elegant  methods  of  adjusting  the  polar 

^  See  also  two  valuable  papers  by  Sir  Howard  Grubb,  The  Observatory,  Vol. 
VII,  pp.  9,  43.    Also  in  Jour.  Roy.  Ast.  Soc.  Canada^  Dec,  1921,  Jan.  1922. 


2^0  THE  TELESCOPE 

axis  to  the  pole  which  depend  on  the  use  of  special  graticules  in 
the  eyepiece,  or  on  auxiliary  devices  applied  to  the  telescope, 
the  general  principle  being  automatically  to  provide  for  setting 
off  the  distance  between  Polaris  and  the  pole  at  the  proper  hour 
angle.  A  beautifully  simple  one  is  that  of  Gerrish  {Poy.  Ast. 
29,  283. 

The  simple  plan  here  outlined  will  generally,  however,  prove 
sufficient  for  ordinary  purposes  and  where  high  precision  is 
necessary  one  has  to  turn  to  the  more  conventional  astronomical 
methods. 

If  one  gives  his  telescope  a  permanent  footing  such  as  is  shown 
in  Fig.  171  adjustment  has  rarely  to  be  repeated.  With  a  pillar 
mount  such  as  we  have  just  now  been  considering  the  instrument 
itself  can  be  taken  in  doors  and  replaced  with  very  slight  risk 
of  disturbing  its  setting,  but  some  provision  must  be  made 
for  sheltering  the  mount. 

A  tarpaulin  is  sometimes  recommended  and  indeed  answers 
well,  particularly  if  a  bag  of  rubber  sheeting  is  drawn  loosely 
over  the  mount  first.  Better  still  is  a  box  cover  of  copper  or 
galvanized  iron  set  over  the  mount  and  closely  fitting  well  down 
over  a  base  clamped  to  the  pillar  with  a  gasket  to  close  the  joint. 

But  the  fact  is  when  one  is  dealing  with  a  fine  instrument  like 
Fig.  168  of  as  much  as  5  inches  aperture,  the  question  of  a  per- 
manent housing  (call  it  observatory  if  you  like)  at  once  comes 
up  and  will  not  down. 

It  is  of  course  always  more  convenient  to  have  the  telescope 
permanently  in  place  and  ready  for  action.  Some  observers 
feel  that  working  conditions  are  better  with  the  telescope  in  the 
open,  but  most  prefer  a  shelter  from  the  wind,  even  if  but  partial, 
and  the  protection  of  a  covering,  however  slight,  in  severe 
weather. 

In  the  last  resort  the  question  is  mainly  one  of  climate.  Where 
nights,  otherwise  of  the  best  seeing  quality,  are  generally  windless 
or  with  breezes  so  slight  that  the  tube  does  not  quiver  a  telescope 
in  the  open,  however  protected  between  times,  works  perfectly 
well. 

In  other  regions  the  clearest  nights  are  apt  to  be  those  of  a 
steady  gentle  wind  producing  great  uniformity  of  conditions  at  the 
expense  of  occasional  vibration  of  the  instrument  and  of  discom- 
fort to  the  observer.  Hence  one  finds  all  sorts  of  practice,  varied 
too,  by  the  inevitable  question  of  expense. 


SETTING  UP  AND  HOUSING  THE  TELESCOPE 


241 


The  simplest  possible  housing  is  to  provide  for  the  fixed 
instrument  a  moveable  cover  which  can  be  lifted  or  slid  quite 
out  of  the  way  leaving  the  telescope  in  the  open  air,  exposed  to 
wind,  but  free  from  the  disturbing  air  currents  that  play  around 
the  opening  of  a  dome.     Shelters  of  this  cheap  and  simple  sort 


Fig.  173. — The  Simplest  of  Telescope  Housings. 


have  been  long  in  use  both  for  small  and  large  instruments. 
For  example  several  small  astrographic  instruments  in  the 
Harvard  equipment  are  mounted  as  shown  in  Fig.  173.  Here 
are  two  fork  mounts,  each  on  a  short  pier,  and  covered  in  by 
galvanized  iron  hoods  made  in  two  parts,  a  vertical  door  which 
swings  down,  as  in  the  camera  of  the  foreground,  and  the  hood 
proper,  hinged  to  the  base  plate  and  free  to  swing  down  when 
the  rear  door  is  unlocked  and  opened.     A  little  to  the  rear  is  a 


242 


THE  TELESCOPE 


similar  astrographic  camera  with  the  hood  closed.  It  is  all  very 
simple,  cheap,  and  effective  for  an  instrument  not  exceeding  say 
two  or  three  feet  in  focal  length. 

A  very  similar  scheme  has  been  successfully  tried  on  reflectors 
as  shown  in  Fig.  174.  The  instrument  shown  is  a  Browning 
equatorial  of  Sj^^  inches  aperture.  The  cover  is  arranged  to 
open  after  the  manner  of  Fig.  173  and  the  plan  proved  very 
effective,  preserving  much  greater  uniformity  of  conditions  and 

hence  permitting  better  defini- 
tion than  in  case  of  a  similar 
instrument  peering  through  the 
open  shutter  of  a  dome. 

Such  a  contrivance  gets  un- 
wieldly  in  case  of  a  refractor  on 
account  of  the  more  considerable 
height  of  the  pier  and  the  length 
of  the  tube  itself.  But  a  modi- 
fication of  it  may  be  made  to 
serve  exceedingly  well  in  climates 
where  working  in  the  open  is 
advantageous.  A  good  example 
is  the  equatorial  of  the  Harvard 
Observatory  station  at  Mande- 
ville,  Jamaica,  which  has  been 
thus  housed  for  some  twenty  years,  as  shown  in  Fig.  175. 

This  11  inch  refractor,  used  mainly  on  planetary  detail,  is 
located  alongside  the  polar  telescope  of  12  inches  aperture  and 
135  feet  4  inches  focal  length  used  for  making  a  photographic 
atlas  of  the  moon  and  on  other  special  problems.  The  housing, 
just  big  enough  to  take  in  the  equatorial  with  the  tube  turned 
low,  opens  on  the  south  side  and  then  can  be  rolled  northward  on 
its  track,  into  the  position  shown,  where  it  is  well  clear  of  the 
instrument,  which  is  then  ready  for  use. 

The  climate  of  Jamaica,  albeit  extremely  damp,  affords  remark- 
ably good  seeing  during  a  large  part  of  the  year,  and  permits 
use  of  the  telescope  quite  in  the  open  without  inconvenience  to 
the  observer.  The  success  of  this  and  all  similar  housing  plans 
depends  on  the  local  climate  more  than  on  anything  else — chiefly 
on  wind  during  the  hours  of  good  seeing.  An  instrument  quite 
uncovered  suffers  from  gusts  far  more  than  one  housed  under  a 
dome,  which  is  really  the  sum  of  the  whole  matter,  save  that  a 


Fig.  174. — Cover  for  Small  Reflector. 


SETTING  UP  AND  HOUSING  THE  TELESCOPE 


243 


dome  to  a  slight  extent  does  shelter  the  observer  in  extremely 
cold  weather. 

Even  very  large  reflectors  can  be  housed  in  similar  fashion 
if  suitably  mounted.  For  example  in  Fig.  176  is  shown  the  36 
inch  aperture  reflector  of  the  late  Dr.  Common,  which  was 
fitted  with  an  open  fork  equatorial  mounting.  Here  the  telescope 
itself,  with  its  short  pier  and  forked  polar  axis,  is  shown  in  dotted 
lines. 


Fig.  175. — Sliding  Housing  for  11-inch  Refractor. 


Built  about  it  is  a  combined  housing  and  observing  stand 
rotatable  on  wheels  T  about  a  circular  track  R.  The  housing 
consists  of  low  corrugated  metal  sides  and  ends,  here  shown 
partly  broken  away,  of  dimensions  just  comfortably  sufficient 
to  take  in  the  telescope  when  the  housing  is  rotated  to  the  north 
and  south  position,  and  the  tube  turned  down  nearly  flat  south- 
ward. A  well  braced  track  WW  extends  back  along  the  top  of 
the  side  housing  and  well  to  the  rear.  On  this  track  rolls  the 
roof  of  the  housing  X,X,X,  with  a  shelter  door  at  the  front  end. 


244 


THE  TELESCOPE 


SETTING  UP  AND  HOUSING   THE  TELESCOPE 


245 


The  members  U  constitute  a  framing  which  supports  at  once 
the  housing  and  the  observing  platform,  to  which  access  is  had 
by  a  ladder,  Z,  provided  with  a  counter-balanced  observing  seat. 
The  instrument  is  put  into  action  by  clearing  the  door  at  the 
end  of  the  roof,  running  the  roof  back  to  the  position  shown  in 
the  dotted  lines,  raising  the  tube,  and  then  revolving  the  whole 
housing  into  whatever  position  is  necessary  to  permit  the  proper 
setting  of  the  tube. 


I'lG.  1 1  7.-- Sliding  Ivouf  Obdervalor\-. 

This  arrangement  worked  well  but  was  found  a  bit  troublesome 
owing  to  wind  and  weather.  With  a  skeleton  tube  and  in  a 
favorable  climate  the  plan  would  succeed  admirably  providing 
an  excellent  shelter  for  a  large  telescope  at  very  low  cost. 

Since  a  fork  mount  allows  the  tube  to  lie  flat,  such  an  instru- 
ment, up  to  say  8  or  10  inches  aperture  can  be  excellently  pro- 
tected by  covers  fitting  snugly  upon  a  base  and  hght  enough  to 
lift  off  as  a  whole. 

The  successful  use  of  all  these  shelters  however  depends  on 
climatic  conditions.  They  require  circumstances  allowing  obser- 
vation in  the  open,  as  with  tripod  mounts,  and  afford  no  protec- 


246  THE  TELESCOPE 

tion  from  wind  or  cold.  Complete  protection  for  the  observer 
cannot  be  had,  except  by  some  of  the  devices  shown  in  Chapter 
V,  but  conditions  can  be  improved  by  permanent  placement  in 
an  observatory,  simple  or  elaborate,  as  the  builder  may  wish. 

The  word  observatory  may  sound  formidable,  but  a  modest 
one  can  be  provided  at  less  expense  than  a  garage  for  the  humblest 
motor  car.  The  chief  difference  in  the  economic  situation  is 
that  not  even  the  most  derided  car  can  be  picked  up  and  carried 
into  the  back  hall  for  shelter,  and  it  really  ought  not  to  be  left 
out  in  the  weather. 

The  next  stage  of  evolution  is  the  telescope  house  with  a 
sliding  roof  in  one  or  more  sections — ordinarily  two.  In  this 
case  the  building  itself  is  a  simple  square  structure  large  enough 
to  accommodate  the  instrument  with  maneuvering  room  around 
it.  The  side  walls  are  carried  merely  high  enough  to  give  clear- 
ance to  the  tube  when  turned  nearly  fiat  and  to  give  head  room 
to  the  observer.  The  roof  laps  with  a  close  joint  in  the  middle 
and  each  half  rolls  on  a  track  supported  beyond  the  ends  of  the 
building  by  an  out-rigger  arranged  in  any  convenient  manner. 

When  the  telescope  is  in  use  the  roof  sections  are  displaced 
enough  to  give  an  ample  clear  space  for  observing,  often  wide 
open  as  shown  in  Fig.  177,  which  is  the  house  of  the  16  inch  Met- 
calf  photographic  doublet  at  the  Harvard  Observatory.  This 
instrument  is  in  an  open  fork  mount  like  that  shown  in  Fig.  139. 

The  sliding  roof  type  is  on  the  whole  the  simplest  structure 
that  can  be  regarded  as  an  observatory  in  the  sense  of  giving 
some  shelter  to  the  observer  as  well  as  the  instrument.  It  gives 
ample  sky  room  for  practical  purposes  even  to  an  instrument 
with  a  fork  mount,  since  in  most  localities  the  seeing  within 
30°  or  so  of  the  horizon  is  decidedly  bad.  If  view  nearer  the 
horizon  is  needed  it  can  readily  be  secured  by  building  up  the 
pier  a  bit. 

Numberless  modifications  of  the  sliding  roof  type  will  suggest 
themselves  on  a  little  study.  One  rather  interesting  one  is  used 
in  the  housing  of  the  24  inch  reflector  of  the  Harvard  Observatory, 
11  feet  3  inches  in  focal  length,  the  same  of  which  the  drive  in  its 
original  dome  is  shown  in  Fig.  139.  As  now  arranged  the  lower 
part  of  the  observatory  remains  while  the  upper  works  are  quite 
similar  in  principle  to  the  housing  of  Dr.  Common's  3  foot  reflector 
of  Fig.  176.  The  cover  open  is  shown  in  Fig.  178.  It  will  be 
seen  that  on  the  north  side  of  the  observatory  there  is  an  outrigger 


SETTING  UP  AND  HOUSING  THE  TELESCOPE 


247 


on  which  the  top  housing  sHdes  clear  of  the  low  revolving  turret 
which  gives  access  to  the  ocular  fitting  used  generally  to  carry  the 
plate  holder,  and  the  eyepiece  for  following  when  required. 
The  tube  cannot  be  brought  to  the  horizontal,  but  it  easily 
commands  all  the  sky-space  that  can  advantageously  be  used 
in  this  situation,  and  the  protection  given  the  telescope  when 
not  in  use  is  very  complete.  To  close  the  observatory  the  tube 
is  brought  north  and  south  and  turned  low  and  the  sliding  roof 
is  then  run  back  into  its  fixed  position.  The  turret  is  very  easily 
turned  by  hand. 


Fig.  178.— Turret  Housing  of  the  24-inch  HaVard  Reflector. 


Of  course  for  steady  work  with  the  maximum  shelter  for 
observer  obtainable  without  turning  to  highly  special  types  of 
housing,  the  familiar  dome  is  the  astronomer's  main  reliance. 
It  is  in  the  larger  sizes  usually  framed  in  steel  and  covered  with 
wood,  externally  sheathed  in  copper  or  steel.  Sometimes  in  smaller 
domes  felt  covered  with  rubberoid  serves  a  good  purpose,  and 
painted  canvas  is  now  and  then  used,  with  wooden  framing. 

But  even  the  smallest  dome  of  conventional  construction  is 
heavy  and  rather  expensive,  and  for  home  talent  offers  many 
difficulties,  especially  with  respect  to  the  shutter  and  shutter 


248 


THE  TELESCOPE 


opening.     A  hemisphere  is  neither  easy  to  frame  nor  to  cover, 
and  the  curved  sHding  shutter  is  especially  troublesome. 

Hence  for  small  observatories  other  forms  of  revolving  roof 
are  desirable,  and  quite  the  easiest  and  cheapest  contrivance  is 
that  embodied  in  the  "Romsey"  type  of  observatory,   devised 


Fig.  179. — The  Original  "Romsey"  Observatory. 

half  a  century  ago  by  that  accomplished  amateur  the  Rev.  E.  L. 
Berthon,  vicar  of  Romsey.  The  feature  of  his  construction  is 
an  unsymmetrical  peak  in  the  revolving  roof  which  permits  the 
ordinary  shutter  to  be  replaced  by  a  hinged  shutter  like  the 
skylight  in  a  roof,  exposing  the  sky  beyond  the  zenith  when 
open,  and  closing  down  over  a  coaming  to  form  a  water  tight 
joint. 


SETTING  UP  AND  HOUSING  THE  TELESCOPE  249 

Berthon's  original  description  of  his  observatory,  which  accom- 
modated a  93^^  inch  reflector,  may  be  found  in  Vol.  14  of  the 
English  Mechanic  and  World  of  Science  whence  Fig.  179  is  taken. 
In  this  plate  Fig.  1  shows  the  complete  elevation  and  Fig.  2 
the  ground  plan,  each  to  a  scale  of  a  eighth  of  an  inch  to  the  foot. 
In  the  plan,  A, A,  are  the  main  joists,  P  the  pier  for  the  telescope, 
T  that  for  the  transit,  and  C  the  clock.  Figs.  3,  4,  and  5  are  of 
details.  In  the  last  named  A  is  a  rafter,  b  the  base  ring,  c  the 
plate,  d  one  of  the  sash  rollers  carrying  the  roof,  and  e  a  lateral 
guide  roller  holding  the  roof  in  place. 

The  structure  can  readily  be  built  without  the  transit  shelter, 
and  in  fact  now-a-days  most  observers  find  it  easier  to  pick  up 
their  time  by  wireless.  The  main  bearing  ring  is  cut  out  of 
ordinary  J-^  inch  board,  in  ten  or  a  dozen,  or  more,  sections 
according  to  convenience,  done  in  duplicate,  joints  lapping,  and 
put  very  firmly  together  with  screws  set  up  hard.  Sometimes 
3  layers  are  thus  used. 

The  roof  in  the  original  "Romsey"  observatory  was  of  painted 
canvas,  but  rubberoid  or  galvanized  iron  lined  with  roofing 
paper  answers  well.  The  shutter  can  be  made  single  or  double 
in  width,  and  counterbalanced  if  necessary.  The  framing  may 
be  of  posts  set  in  the  ground  as  here  shown,  or  with  sills  resting 
on  a  foundation,  and  the  walls  of  any  construction — matched 
boards  of  any  kind,  cement  on  wire  lath,  hollow  tile,  or  concrete 
blocks. 

Chambers'  Handbook  of  Astronomy  Vol.  II  contains  quite 
complete  details  of  the  "Romsey"  type  of  observatory  and  is 
easier  to  get  at  than  tne  original  description. 

A  very  neat  adaptation  of  the  plan  is  shown  in  Fig.  180,  of 
which  a  description  may  be  found  in  Popular  Astronomy  28,  183. 
This  observatory  was  about  9  feet  in  diameter,  to  house  a  4 
inch  telescope,  and  was  provided  with  a  rough  concrete  founda- 
tion on  which  was  built  a  circular  wall  6  feet  high  of  hollow  glazed 
tile,  well  levelled  on  top.  To  this  was  secured  a  ring  plate  built 
up  in  two  layers,  carrying  two  circles  of  wooden  strips  with  a 
couple  of  inches  space  between  them  for  a  run-way.  In  this 
ran  6  two-inch  truck  castors  secured  to  a  similar  ring  plate 
on  which  was  built  up  the  frame  of  the  "  dome"  arranged 
as  shown.  Altogether  a  very  neat  and  workmanlike  affair, 
in  this  case  built  largely  by  the  owner  but  permitting  construc- 
tion at  very  small  expense  almost  anywhere.     Another  interesting 


250 


THE  TELESCOPE 


modification  of  the  same  general  plan  in  the  same  volume  just 
cited  is  shown  in  Fig.  181.  This  is  also  for  a  4  inch  refractor  and 
the  dome  proper  is  but  8  feet  4  inches  in  diameter.  Like  the 
preceding  structure  the  foundation  is  of  concrete  but  the  walls  are 
framed  in  spruce  and  sheathed  in  matched  boards  with  a  "beaver- 
board"  lining. 


Fig.  180. — A  More  Substantial  "Romsey"  Type. 


The  ring  plate  is  three-ply,  12  sections  to  the  layer,  and  its 
mate  on  which  the  dome  is  assembled  is  similarly  formed,  though 
left  with  the  figure  of  a  dodecagon  to  match  the  dome.  The 
weight  is  carried  on  four  rubber  tired  truck  rollers,  and  there 
are  lateral  guide  rollers  on  the  plan  of  those  in  Fig.  179. 

The  dome  itself  however,  is  wholly  of  galvanized  iron,  in  12 
gores  joined  with  standing  seams,  turned,  riveted,  and  soldered. 


SETTING   UP  AND  HOUSING  THE  TELESCOPE 


251 


■ 

r^ 

■ 

<s 

252  THE  TELESCOPE 

There  is  a  short  shutter  at  the  zenith  sHding  back  upon  a  frame, 
while  the  main  shutter  is  removed  from  the  outside  by  handles. 

Observatories  of  the  Romsey  or  allied  types  can  be  erected 
at  very  moderate  cost,  varying  considerably  from  place  to  place, 
but  running  at  present  say  from  $200  to  $600,  and  big  enough  to 
shelter  refractors  of  4  to  6  inches  aperture.  The  revolving  roofs 
will  range  from  9  to  12  feet  in  diameter.  If  reflectors  are  in  use, 
those  of  about  double  these  apertures  can  be  accommodated 
since  the  reflector  is  ordinarily  much  the  shorter  for  equal 
aperture. 

The  sliding  roof,  not  to  say  the  sliding  shelter,  forms  of  housing 
cost  somewhat  less,  depending  on  the  construction  adopted. 
Going  to  brick  may  double  the  figures  quoted,  but  such  solidity 
is  generally  quite  needless,  though  it  is  highly  desirable  that  the 
cover  of  a  valuable  instrument  should  be  fire-proof  and  not  easily 
broken  open.  The  stealing  of  objectives  and  accessories  is  not 
unknown,  and  vandalism  is  a  risk  not  to  be  forgotten.  But  to 
even  the  matter  up,  housing  a  telescope  is  rather  an  easy  thing 
to  accomplish,  and  as  a  matter  of  fact  for  the  price  of  a  very 
modest  motor  car  one  can  both  buy  and  house  an  instrument 
big  enough  to  be  of  genuine  service. 


CHAPTER  XI 
SEEING  AND  MAGNIFICATION 

Few  things  are  more  generally  disappointing  than  one's  first 
glimpse  of  the  Heavens  through  a  telescope.  The  novice  is  fed 
up  with  maps  of  Mars  as  a  great  disc  full  of  intricate  markings, 
and  he  generally  sees  a  little  wriggling  ball  of  light  with  no  more 
visible  detail  than  an  egg.  It  is  almost  impossible  to  believe 
that,  at  a  fair  opposition,  Mars  under  the  power  of  even  the 
smallest  astronomical  telescope  really  looks  as  big  as  the  full  moon. 
Again,  one  looks  at  a  double  star  to  see  not  two  brilliant  little 
discs  resplendent  in  color,  but  an  indeterminate  flicker  void  of 
shape  and  hue. 

The  fact  is,  that  most  of  the  time  over  most  of  the  world 
seeing  conditions  are  bad,  so  that  the  telescope  does  not  have  a 
fair  chance,  and  on  the  whole  the  bigger  the  telescope  the  worse 
the  chance.  One  famous  English  astronomer,  possessed  of  a 
fine  refractor  that  would  be  reckoned  large  even  now-a-days, 
averred  that  he  had  seen  but  one  first  class  night  in  fifteen 
years  past. 

The  case  is  really  much  less  bad  than  this  implies,  for  even  in 
rather  unfavorable  climates  many  a  night,  at  some  o'clock  or 
other,  will  furnish  an  hour  or  two  of  pretty  good  seeing,  while 
now  and  then,  without  any  apparent  connection  with  the  pre- 
vious state  of  the  weather,  a  night  will  turn  up  when  the  pictures 
in  the  popular  astronomies  come  true,  the  stars  shrink  to  steady 
points  set  in  clean  cut  rings,  and  no  available  power  seems  too 
high. 

One  can  get  a  good  idea  of  the  true  inwardness  of  bad  seeing 
by  trying  to  read  a  newspaper  through  an  opera  glass  across  a 
hot  stove.  If  the  actual  movements  in  the  atmosphere  could  be 
made  visible  they  would  present  a  strange  scene  of  turbulence — 
rushing  currents  taking  devious  courses  up  and  around  obstacles, 
slowly  moving  whirlpools,  upward  slants  such  as  gulls  hug  on 
the  quarter  of  a  liner,  great  downward  rushes  dreaded  by  the 
aviator,  and  over  it  all  incessant  ripples  in  every  direction. 

253 


254  THE  TELESCOPE 

And  movements  of  air  are  usually  associated  with  changes  of 
temperature,  as  over  the  stove,  varying  the  refraction  and  con- 
torting the  rays  that  come  from  a  distant  star  until  the  image  is 
quite  ruined. 

The  condition  for  excellence  of  definition  is  that  the  atmosphere 
through  which  we  see  shall  be  homogeneous,  whatever  its  tem- 
perature, humidity,  or  general  trend  of  movement.  Irregular 
refraction  is  the  thing  to  be  feared,  particularly  if  the  variations 
are  sudden  and  frequent.  Hence  the  common  troubles  near  the 
ground  and  about  buildings,  especially  where  there  are  roofs  and 
chimneys  to  radiate  heat — even  in  and  about  an  observatory  dome. 

Professor  W.  H.  Pickering,  who  has  had  a  varied  experience 
in  climatic  idiosyncrasies,  gives  the  Northern  Atlantic  seaboard 
the  bad  preeminence  of  having  the  worst  observing  conditions 
of  any  region  within  his  knowledge.  The  author  cheerfully 
concurs,  yet  now  and  then,  quite  often  after  midnight,  the  air 
steadies  and,  if  the  other  conditions  are  good,  definition  becomes 
fairly  respectable,  sometimes  even  excellent. 

Temperature  and  humidity  as  such,  seem  to  make  little 
difference,  and  a  steady  breeze  unless  it  shakes  the  instrument  is 
relatively  harmless.  Hence  we  find  the  most  admirable  defini- 
tion in  situations  as  widely  different  as  the  Harvard  station  at 
Mandeville,  Jamaica;  Flagstaff,  Arizona  7000  feet  up  and  snow 
bound  in  winter;  Italy,  and  Egypt.  The  first  named  is  warm  and 
with  very  heavy  rainfall  and  dew,  the  second  dry  with  rather 
large  seasonal  variation  of  temperature,  and  the  others  temperate 
and  hot  respectively. 

Perhaps  the  most  striking  evidence  of  the  importance  of  uni- 
formity was  noted  by  Evershed  at  an  Indian  station  where  good 
conditions  immediately  followed  the  flooding  of  the  rice  fields 
with  its  tendency  to  stabilize  the  temperature.  Mountain 
stations  may  be  good  as  at  Flagstaff,  Mt.  Hamilton,  or  Mt. 
Wilson,  or  very  bad  as  Pike's  Peak  proved  to  be,  probably  owing 
to  local  conditions. 

In  fact  much  of  the  trouble  comes  from  nearby  sources,  atmos- 
pheric waves  and  ripples  rather  than  large  movements,  ripples 
indeed  often  small  compared  with  the  aperture  of  the  telescope 
and  sometimes  in  or  not  far  outside  of  the  tube  itself. 

Aside  from  these  difficulties,  there  are  still  others  which  have 
to  do  with  the  transparency  of  the  atmosphere  with  respect  to  its 
suspended  matter.     This  does  not  affect  the  definition  as  such, 


SEEING  AND  MAGNIFICATION  255 

but  it  cuts  down  the  light  to  a  degree  that  may  interfere  seriously 
with  the  observation  of  faint  stars  and  nebulae.  The  smoke 
near  a  city  aggravates  the  situation,  but  in  particular  it  depends 
on  general  weather  conditions  which  may  be  persistent  or  merely 
temporary. 

Often  seeing  conditions  may  be  admirable  save  for  this  lack 
of  transparency  in  the  atmosphere,  so  that  study  of  the  moon, 
of  planetary  markings  and  even  of  double  stars,  not  too  faint, 
may  go  on  quite  unimpeded.  The  actual  loss  of  light  may  reach 
however  a  magnitude  or  more,  while  the  sky  is  quite  cloudless 
and  without  a  trace  of  fog  or  noticeable  haziness  by  day. 

There  have  been  a  good  many  nights  the  past  year  (1921) 
when  Alcor  (80  Ursae  Majoris)  the  tiny  neighbor  of  Mizar,  very 
nearly  of  the  4th  magnitude,  has  been  barely  or  not  at  all  visible 
while  the  seeing  otherwise  was  respectably  good.  Ordinarily 
stars  of  6""  should  be  visible  in  a  really  clear  night,  and  in  a 
brilliant  winter  sky  in  the  temperate  zones,  or  in  the  clear  air  of 
the  tropics,  a  good  many  eyes  will  do  better  than  this,  reaching 
6™.5  or  even  7™,  occasionally  a  bit  more. 

The  relation  of  air  waves  and  such  like  irregularities  to  tele- 
scopic vision  was  rather  thoroughly  investigated  by  Douglass 
more  than  twenty  years  ago  (Pop.  Ast.  6,  193)  with  very  interest- 
ing results.  In  substance,  from  careful  observation  with  tele- 
scopes from  4  inches  up  to  24  inches  aperture,  he  found  that  the 
real  trouble  came  from  what  one  may  call  ripples,  disturbances 
from  say  4  inches  wave  length  down  to  %  inch  or  less.  Long 
waves  are  rare  and  relatively  unimportant  since  their  general 
effect  is  to  cause  shifting  of  the  image  as  a  whole  rather  than  the 
destruction  of  detail  which  accompanies  the  shorter  waves. 

This  rippling  of  the  air  is  probably  associated  with  the  contact 
displacements  in  air  currents  such  as  on  a  big  scale  become 
visible  in  cloud  forms.  Clearly  ripples,  marked  as  they  are  by 
difference  of  refraction,  located  in  front  of  a  telescope  objective, 
produce  different  focal  lengths  for  different  parts  of  the  objective 
and  render  a  clean  and  stable  image  quite  out  of  the  question. 

In  rough  terms  Douglass  found  that  waves  of  greater  length 
than  half  the  aperture  did  not  materially  deteriorate  the  image, 
although  they  did  shift  it  as  a  whole,  while  waves  of  length  less 
than  one  third  the  aperture  did  serious  mischief  to  the  definition, 
the  greater  as  the  ripples  were  shorter,  and  the  image  itself  more 
minute  in  dimension  or  detail. 


256  THE  TELESCOPE 

Hence  there  are  times  when  decreasing  the  aperture  of  an 
objective  by  a  stop  improves  the  seeing  considerably  by  increasing 
the  relative  length  of  the  air  waves.  Such  is  in  fact  found  to  be 
the  case  in  practical  observing,  especially  when  the  seeing  with  a 
large  aperture  is  decidedly  poor.  In  other  words  one  may  often 
gain  more  by  increased  steadiness  than  he  loses  by  lessened 
"resolving  power/'  the  result  depending  somewhat  on  the  class 
of  observation  which  chances  to  be  under  way. 

And  this  brings  us,  willy-nilly,  to  the  somewhat  abstruse 
matter  of  resolving  power,  depending  fundamentally  upon  the 
theory  of  diffraction  of  light,  and  practically  upon  a  good  many 
other  things  that  modify  the  character  of  the  diffraction  pattern, 
or  the  actual  visibility  of  its  elements. 

When  light  shines  through  a  hole  or  a  slit  the  light  waves  are 
bent  at  the  margins  and  the  several  sets,  eventually  overlapping, 
interfere  with  each  other  so  as  to  produce  a  pattern  of  bright  and 
dark  elements  depending  on  the  size  and  shape  of  the  aperture, 
and  distributed  about  a  central  bright  image  of  that  aperture. 
One  gets  the  effect  well  in  looking  through  an  open  umbrella  at  a 
distant  street  light.  The  outer  images  of  the  pattern  are  fainter 
and  fainter  as  they  get  away  from  th^isentral  image. 

Without  burdening  the  reader  for  the  moment  with  details  to  be 
considered  presently,  the  effect  in  telescopic  vision  is  that  a  star 
of  real  angular  diameter  quite  negligible,  perhaps  0."001  of  arc, 
is  represented  by  an  image  under  perfect  conditions  like  Fig.  154, 
of  'quite  perceptible  diameter,  surrounded  by  a  system  of  rings, 
faint  but  clear-cut,  diminishing  in  intensity  outwards.  When 
the  seeing  is  bad  no  rings  are  visible  and  the  central  disc  is  a 
mere  bright  blur  several  times  larger  than  it  ought  to  be. 

The  varying  appearance  of  the  star  image  is  a  very  good  index 
of  the  quality  of  the  seeing,  so  that,  having  a  clear  indication  of 
this  appearance,  two  astronomers  in  different  parts  of  the  world 
can  gain  a  definite  idea  of  each  other's  relative  seeing  conditions. 
To  this  end  a  standard  scale  of  seeing,  due  largely  to  the  efforts 
of  Prof.  W.  H.  Pickering,  has  come  into  rather  common  use. 
(H.  A.  61  29).  It  is  as  follows,  based  on  observations  with  a 
5  inch  telescope. 

STANDARD  SCALE  OF  SEEING 

1.  Image  usually  about  twice  the  diameter  of  the  third  ring. 

2.  Image  occasionally  twice  the  diameter  of  the  third  ring. 


SEEING  AND  MAGNIFICATION  .  257 

3.  Image  of  about  the  same  diameter  as  the  third  ring,  and 
brighter  at  the  centre. 

4.  Disc  often  visible,  arcs  (of  rings)  sometimes  seen  on  brighter 
stars. 

5.  Disc  always  visible,  arcs  frequently  seen  on  brighter  stars. 

6.  Disc  always  visible,  short  arcs  constantly  seen. 

7.  Disc  sometimes  sharply  defined,  (a)  long  arcs.  (&)  Rings 
complete. 

8.  Disc  always  sharply  defined,  (a)  long  arcs.  (6)  Rings  com- 
plete all  in  motion. 

9.  Rings,  (a)  Inner  ring  stationary,  (6)  Outer  rings  momen- 
tarily stationary. 

10.  Rings  all  stationary,  (a)  Detail  between  the  rings  some- 
times moving.     (6)  No  detail  between  the  rings. 

The  first  three  scale  numbers  indicate  very  bad  seeing;  the 
next  two,  poor;  the  next  two,  good;  and  the  last  three,  excellent. 
One  can  get  some  idea  of  the  extreme  badness  of  scale  divisions 
1,  2,  3,  in  realizing  that  the  third  bright  diffraction  ring  is  nearly 
4  times  the  diameter  of  the  proper  star-disc. 

It  must  be  noted  that  for  a  given  condition  of  atmosphere  the 
seeing  with  a  large  instrument  ranks  lower  on  the  scale  than  with 
a  small  one,  since  as  already  explained  the  usual  air  ripples 
are  of  dimensions  that  might  affect  a  5  inch  aperture  imper- 
ceptibly and  a  15  inch  aperture  very  seriously. 

Douglass  (  loc.  cit.)  made  a  careful  comparison  of  seeing  con- 
ditions for  apertures  up  to  24  inches  and  found  a  systematic 
difference  of  2  or  3  scale  numbers  between  4  or  6  inches  aperture, 
and  18  or  24  inches.  With  the  smallest  aperture  the  image 
showed  merely  bodily  motion  due  to  air  waves  that  produced 
serious  injury  to  the  image  in  the  large  apertures,  as  might  be 
expected. 

There  is  likewise  a  great  difference  in  the  average  quality  of 
seeing  as  between  stars  near  the  zenith  and  those  toward  the 
horizon,  due  again  to  the  greater  opportunity  for  atmospheric 
disturbances  in  the  latter  case.  Pickering's  experiments  (loc. 
cit.)  show  a  difference  of  nearly  3  scale  divisions  between  say 
20°  and  70°  elevation.  This  difference,  which  is  important,  is 
well  shown  in  Fig.  182,  taken  from  his  report. 

The  three  lower  curves  were  from  Cambridge  observations, 
the  others  obtained  at  various  Jamaica  stations.  They  clearly 
show  the  systematic  regional  differences,  as  well  as  the  rapid 


258 


THE  TELESCOPE 


falling  off  in  definition  below  altitude  40°,  which  points  the  impor- 
tance of  making  provision  for  comfortable  observing  above  this 
altitude. 


0°     10°    20'   30°   40°    50°    60°    70°   80*   90* 


1.0 
0.9 
0.8 

0.7 

>> 
.^0.6 

s 
o 

■M 

^0.5 
> 

-S,  0.4 

« 

0.3 
0.2 
0.1 
0.0 


Fig.  182. — Variation  of  Seeing  with  Altitude. 


^! „!= 

ii- 


12  3  4  5  6  7 

n 

Fig.   183. — Airy's  Diffraction  Pattern. 


The  relation  of  the  diffraction  pattern  as  disclosed  in  the 
moments  of  best  seeing  to  its  theoretical  form  is  a  very  interesting 
one.     The  diffraction  through  a  theoretically  perfect  objective 


SEEING  AND  MAGNIFICATION 


259 


was  worked  out  many  years  ago  by  Sir  George  Airy  who  calcu- 
lated the  exact  distribution  of  the  light  in  the  central  disc  and  the 
surrounding  rings. 

This  is  shown  from  the  centre  outwards  in  Fig.  183,  in  which 
the  ordinates  of  the  curve  represent  relative  intensities  while  the 
abscissae  represent  to  an  arbitrary  scale  the  distances  from  the 
axis.     It  will  be  at  once  noticed  that  the  star  image,  brilliant  at 


Fig.   184. — Diffraction  Solid  for  a  Star. 

its  centre,  sinks,  first  rapidly  and  then  more  slowly,  to  a  mini- 
mum and  then  very  gradually  rises  to  the  maximum  of  the  first 
bright  ring,  then  as  slowly  sinks  again  to  increase  for  the  second 
ring  and  so  on. 

For  unity  brightness  in  the  centre  of  the  star  disc  the  maximum 
brightness  of  the  first  ring  is  0.017,  of  the  second  0.004  and  the 
third  0.0016.  The  rings  are  equidistant  and  the  star  disc  has  a 
radius  substantially  equal  to  the  distance  between  rings.  One's 
vision  does  not  follow  down  to  zero  the  intensities  of  the  rings  or 
of  the  margin  of  the  disc,  so  that  the  latter  has  an  apparent 
diameter  materially  less  than  the  diameter  to  the  first  diffraction 
minimum,  and  the  rings  themselves  look  sharper  and  thinner 
than  the  figure  would  show,  even  were  the  horizontal  scale  much 


260  THE  TELESCOPE 

diminished.  The  eye  does  not  descend  in  the  presence  of  bright 
areas  to  its  final  threshold  of  perception. 

One  gains  a  somewhat  vivid  idea  of  the  situation  by  passing 
to  three  dimensions  as  in  Fig.  184,  the  ''diffraction  solid"  for  a 
star,  a  conception  due  to  M.  Andre  (Mem.  de  I'Acad.  de  Lyon 
30,  49).  Here  the  solid  represents  in  volume  the  whole  light 
received  and  the  height  taken  at  any  point,  the  intensity  at  that 
point. 

A  cross  section  at  any  point  shows  the  apparent  diameter  of 
the  disc,  its  distance  to  the  apex  the  remaining  intensity,  and 
the  volume  above  the  section  the  remaining  total  light.  Sub- 
stantially 85  %  of  the  total  light  belongs  to  the  central  cone,  for 
the  theoretical  distribution. 

Granting  that  the  eye  can  distinguish  from  the  back-ground 
of  the  sky,  in  presence  of  a  bright  point,  only  light  above  a  certain 
intensity,  one  readily  sees  why  the  discs  of  faint  stars  look  small, 
and  why  shade  glasses  are  sometimes  useful  in  wiping  out  the 
marginal  intensities  of  the  solid.  There  are  physiological  factors 
that  alter  profoundly  the  appearance  of  the  actual  star  image, 
despite  the  fact  that  the  theoretical  diffraction  image  for  the 
aperture  is  independent  of  the  star's  magnitude. 

Practically  the  general  reduction  of  illumination  in  the  fainter 
stars  cuts  down  the  apparent  diameters  of  their  discs,  and  reduces 
the  number  of  rings  visible  against  the  background  of  the  sky. 

The  scale  of  the  diffraction  system  determines  the  resolving 
power  of  the  telescope.  This  scale  is  given  in  Airy's  original 
paper  (Cambr.  Phil.  Trans.  1834  p.  283),  from  which  the  angle  a 
to  any  maximum  or  minimum  in  the  ring  system  is  defined  by 

X 

sm  or  =  n  Q 

in  which  X  is  numerically  the  wave  length  of  any  light  considered 
and  B  is  the  radius  of  the  objective. 

We  therefore  see  that  the  ring  system  varies  in  dimension 
inversely  with  the  aperture  of  the  objective  and  directly  with  the 
wave  length  considered.  Hence  the  bigger  the  objective  the 
smaller  the  disc  and  its  surrounding  ring  system;  and  the  greater 
the  wave  length,  i.e.  the  redder  the  light,  the  bigger  the  diffrac- 
tion system.  Evidently  there  should  be  color  in  the  rings  but 
it  very  seldom  shows  on  account  of  the  faintness  of  the 
illumination. 

Now  the  factor  n  is  for  the  first  dark  ring  0.61,  and  for  the  first 


SEEING  AND  MAGNIFICATION  261 

bright  ring  0,81,  as  computed  from  Airy's  general  theory,  and 
therefore  if  we  reckon  that  two  stars  will  be  seen  as  separate 
when  the  central  disc  of  one  falls  on  the  first  dark  ring  of  the 
other  the  angular  distance  will  be 

X 

Smo:  =0.61  ^, 
K 

and,  taking  X  at  the  brightest  part  of  the  spectrum  i.e.,  about 
560  /i/x,  in  the  yellow  green,  with  a  taken  for  sin  a,  we  can 
compute  this  assumed  separating  power  for  any  aperture.  Thus 
560  iJLiJL  being  very  nearly  3^^5,500  inch,  and  assuming  a  5  inch 
telescope,  the  instrument  should  on  this  basis  show  as  double  two 
stars  whose  centres  are  separated  by  l."l  of  arc. 

In  actual  fact  one  can  do  somewhat  better  than  this,  showing 
that  the  visible  diameter  of  the  central  disc  is  in  effect  less  than 
the  diameter  indicated  by  the  diffraction  pattern,  owing  to  the 
reasons  already  stated.  Evidently  the  brightness  of  the  star  is  a 
factor  in  the  situation  since  if  very  bright  the  disc  gains  apparent 
size,  and  when  very  faint  there  is  sufficient  difficulty  in  seeing 
one  star,  let  alone  a  pair. 

The  most  thorough  investigation  of  this  matter  of  resolving 
power  was  made  by  the  Rev.  W.  R.  Dawes  many  years  ago 
(Mem,  R.A.S.  35,  158).  His  study  included  years  of  observa- 
tion with  telescopes  of  different  sizes,  and  his  final  result  was  to 
establish  what  has  since  been  known  as  "Dawes'  Limit." 

To  sum  up  Dawes'  results  he  established  the  fact  that  on  the 
average  a  one  inch  aperture  would  enable  one  to  separate  two 
6th  magnitude  stars  the  centers  of  which  were  separated  by  4.56". 
Or,  to  generalize  from  this  basis,  the  separating  power  of  any 
telescope   is   for   very   nearly   equal   stars,    moderately   bright, 

4". 56 

— 2 —  where  A  is  the  aperture  of  the  telescope  in  inches. 

Many  years  of  experience  have  emphasized  the  usefulness  of 
this  approximate  rule,  but  that  it  is  only  approximate  must  be 
candidly  admitted.  It  is  a  limit  decidedly  under  that  just 
assigned  on  the  basis  of  the  theory  of  diffraction  for  the  central 
bright  wave-lengths  of  the  spectrum.  Attempts  have  been 
made  to  square  the  two  figures  by  assuming  in  the  diffraction 
theory  a  wave  length  of  ^-^SjOOO  inch,  but  this  figure  corresponds 
to  a  point  well  up  into  the  blue,  of  so  low  luminosity  that  it  is  of 
no  importance  whatever  in  the  visual  use  of  a  telescope. 

The  fact  is  that  the  visibility  of  two  neighboring  bright  points 


262 


THE  TELESCOPE 


as  distinct,  depends  on  a  complex  of  physical  and  physiological 
factors,  the  exact  relations  of  which  have  never  been  unravelled. 
To  start  with  we  have  the  principles  of  diffraction  as  just 
explained,  which  define  the  relation  of  the  stellar  disc  to  the 
center  of  the  first  dark  ring,  but  we  know  that  under  no  circum- 
stances can  one  see  the  disc  out  to  this  limit,  since  vision  fails 
to  take  cognizance  of  the  faint  rim  of  the  image.  The  apparent 
diameter  of  the  diffraction  solid  therefore  corresponds  to  a 
section  taken  some  distance  above  the  base,  the  exact  point 
depending  on  the  sensitiveness  of  the  particular  observer's  eye, 
the  actual  brilliancy  of  the  center  of  the  disc,  and  the  corre- 
sponding factors  for  the  neighboring  star. 


Fig.   185. — Diffraction  Solid  for  a  Disc. 

Under  favorable  circumstances  one  would  not  go  far  amiss  in 
taking  the  visible  diameter  of  the  disc  at  about  half  that  reckoned 
to  the  center  of  the  first  dark  ring.  This  figure  in  fact  corre- 
sponds to  what  has  been  shown  to  be  within  the  grasp  of  a  good 
observer  under  favorable  conditions,  as  we  shall  presently  see. 

On  the  other  hand,  if  the  stars  are  decidedly  bright  there  is 
increase  of  apparent  diameter  of  the  disc  due  to  the  phenomenon 
known  as  irradiation,  the  spreading  of  light  about  its  true  image 
on  the  retina  which  corresponds  quite  closely  to  the  halation 
produced  by  a  bright  spot  on  a  photographic  plate. 

If,  on  the  contrary,  the  stars  are  very  faint  the  total  amount  of 
light  available  is  not  sufficient  to  make  contrast  over  and  above 
the  background  sufficient  to  disclose  the  two  points  as  separate, 
while  if  the  pair  is  very  unequal  the  brighter  one  will  produce 
sufficient  glare  to  quite  over-power  the  light  from  the  smaller 
one  so  that  the  eye  misses  it  entirely. 

A  striking  case  of  this  is  found  in  the  companion  to  Sirius, 
an  extremely  difficult  object  for  ordinary  telescopes  although  the 
distance  to  the  companion  is  about  10.6"  and  its  magnitude 


SEEING  AND  MAGNIFICATION  263 

is  8.4,  making  a  superlatively  easy  double  for  the  very  smallest 
telescope  save  for  the  overpowering  eifect  of  the  light  of  the 
large  star.  Another  notoriously  difficult  object  for  small 
telescopes  is  8  Cygni,  a  beautiful  double  of  which  the  smaller 
component  falls  unpleasantly  near  the  first  diffraction  maximum 
of  the  primary  in  which  it  is  apt  to  be  lost. 

"Dawes'  Limit"  is  therefore  subject  to  many  qualifying  fac- 
tors. Lewis,  in  the  papers  already  referred  to  (Obs.  37,  378) 
did  an  admirable  piece  of  investigation  in  going  through  the 
double  star  work  of  about  two  score  trained  observers  working 
with  telescopes  all  the  way  from  4  inches  to  36  inches  aperture. 

From  this  accumulation  of  data  several  striking  facts  stand 
out.  First  there  is  great  difference  between  individual  observers 
working  with  telescopes  of  similar  aperture  as  respects  their 
agreement  with  ''Dawes'  Limit,"  showing  the  effect  of  variation 
in  the  physiological  factors  as  well  as  instrumental  ones. 

Second,  there  is  also  a  very  large  difference  between  the 
facility  of  observing  equal  bright  pairs  and  equal  faint  pairs,  or 
unequal  pairs  of  any  kind,  again  emphasizing  the  physiological 
as  well  as  the  physical  factors. 

Finally,  there  is  most  unmistakable  difference  between  small 
and  large  apertures  in  their  capacity  to  work  up  to  or  past  the 
standard  of  "Dawes'  Limit."  The  smaller  telescopes  are  clearly 
the  more  efficient  as  would  be  anticipated  from  the  facts  just 
pointed  out  regarding  the  different  effect  of  the  ordinary  and 
inescapable  atmospheric  waves  on  small  and  large  instruments. 

The  big  telescopes  are  unquestionably  as  good  optically 
speaking  as  the  small  ones  but  under  the  ordinary  working  condi- 
tions, even  as  good  as  those  a  double  star  observer  seeks,  the 
smaller  aperture  by  reason  of  less  disturbance  from  atmospheric 
factors  does  relatively  much  the  better  work,  however  good  the 
big  instrument  may  be  under  exceptional  conditions. 

This  is  admirably  shown  by  the  discussion  of  the  beautiful 
v/ork  of  the  late  Mr.  Burnham,  than  whom  probably  no  better 
observer  of  doubles  has  been  known  to  astronomy.  His  records 
of  discovery  with  telescopes  of  6,  9.4,  12,  18}^  and  36  inches 
show  the  relative  ease  of  working  to  the  theoretical  limit  with 
instruments  not  seriously  upset  by  ordinary  atmospheric  waves. 

With  the  6  inch  aperture  Burnham  reached  in  the  average  0.53 
of  Dawes'  limit,  quite  near  the  rough  figure  just  suggested,  and 
he  also  fell  well  inside  Dawes'  limit  with  the  9.4  inch  instrument. 


204  THE  TELESCOPE 

With  none  of  the  others  did  he  reach  it  and  in  fact  fell  short  of 
it  by  15  to  60%.  All  observations  being  by  the  same  notably 
skilled  observer  and  representing  discoveries  of  doubles,  so  that  no 
aid  could  have  been  gained  by  familiarity,  the  issue  becomes  ex- 
ceedingly plain  that  size  with  all  its  advantages  in  resolving  power 
brings  serious  countervailing  limitations  due  to  atmosphere. 

But  a  large  aperture  has  besides  its  possible  separating  power 
one  advantage  that  can  not  be  discounted  in  "light  grasp,"  the 
power  of  discerning  faint  objects.  This  is  the  thing  in  which  a 
small  telescope  necessarily  fails.  The  "light  grasp"  of  the 
telescope  obviously  depends  chiefly  on  the  area  of  the  objective, 
and  visually  only  in  very  minor  degree  on  the  absorption  of  the 
thicker  glass  in  the  case  of  a  large  lens. 

According  to  the  conventional  scale  of  star  magnitudes  as  now 
in  universal  use,  stars  are  classified  in  magnitudes  which  differ 
from  each  other  by  a  light  ratio  of  2.512,  a  number  the  logarithm 
of  which  is  0.4,  a  relation  suggested  by  Pogson  some  forty  years 
ago.  A  second  magnitude  star  therefore  gives  only  about  40% 
of  the  light  of  a  first  magnitude  star,  while  a  third  magnitude  star 
gives  again  a  little  less  than  40%  of  the  light  of  a  second  magni- 
tude star  and  so  on. 

But  doubling  the  aperture  of  a  telescope  increases  the  avail- 
able area  of  the  objective  four  times  and  so  on,  the  "light  grasp" 
being  in  proportion  to  the  square  of  the  aperture.  Thus  a  10 
inch  objective  will  take  in  and  deliver  nearly  100  times  as  much 
light  as  would  a  1  inch  aperture.  If  one  follows  Pogson's  scale 
down  the  line  he  will  find  that  this  corresponds  exactly  to  5 
stellar  magnitudes,  so  that  if  a  1  inch  aperture  discloses,  as  it 
readily  does,  a  9th  magnitude  star,  a  10  inch  aperture  should 
disclose  a  14th  magnitude  star. 

Such  is  substantially  in  fact  the  case,  and  one  can  therefore 
readily  tabulate  the  minimum  visible  for  an  aperture  just  as  he 
can  tabulate  the  approximate  resolving  power  by  reference  to 
Dawes'  limit.  Fig.  186  shows  in  graphic  form  both  these  rela- 
tions for  ready  reference,  the  variation  of  resolving  power  with 
aperture,  and  that  of  "light  grasp,"  reckoned  in  stellar 
magnitudes. 

It  is  hardly  necessary  to  state  that  considerable  individual  and 
observational  differences  will  be  found  in  each  of  these  cases, 
in  the  latter  amounting  to  not  less  than  0.5  to  1.0  magnitude 
either  way.     The  scale  is  based  on  the  9th  magnitude  star  just 


SEEING  AND  MAGNIFICATION 


265 


being  visible  with  1  inch  aperture,  whereas  in  fact  under  varying 
conditions  and  with  various  observers  the  range  may  be  from  the 
8th  to  10th  magnitude.  All  these  things,  however  convenient, 
must  be  taken  merely  at  their  true  value  as  good  working 
approximations. 

Even  the  diffraction  theory  can  be  taken  only  as  an  approxi- 
mation since  no  optical  surface  is  absolutely  perfect  and  in 
the  ordinary  refracting  telescope  there  is  a  necessary  residual 
chromatic  aberration  beside  whatever  may  remain  of  spherical 
errors. 


I- 

\ 

\ 

^ 

'^ 

/ 

^ 

\ 

CO 

2» 

/ 

\ 

\ 

\ 

\. 

^s. 

"^ 

_  c! 


0  2  4  6  8  10  12  14  l(j  18  20  22  24. 

Aperture  in  Inches 

Fig.   186. — Light-grasp  and    Resolving  Power. 

It  is  a  fact  therefore,  as  has  been  shown  by  Conrady  (M.N. 
79  575)  following  up  a  distinguished  investigation  by  Lord 
Rayleigh  (Sci.  Papers  1  415),  that  a  certain  small  amount  of 
aberration  can  be  tolerated  without  material  effect  on  the 
definition,  which  is  very  fortunate  considering  that  the  secondary 
spectrum  represents  aberrations  of  about  3^^,000  of  the  focal 
length,  as  we  have  already  seen. 

The  chief  effect  of  this,  as  of  very  slight  spherical  aberration, 
is  merely  to  reduce  the  maximum  intensity  of  the  central  disc 
of  the  diffraction  pattern  and  to  produce  a  faint  haze  about  it 
which  slightly  illuminates  the  diffraction  minima.     The  visible 


266  THE  TELESCOPE 

diameter  of  the  disc  and  the  relative  distribution  of  intensity  in 
it  is  not  however  materially  changed  so  that  the  main  effect  is  a 
little  loss  and  scattering  of  light. 

With  larger  aberrations  these  effects  are  more  serious  but 
where  the  change  in  length  of  optical  path  between  the  ray 
proceeding  through  the  center  of  the  objective  and  that  from  the 
margin  does  not  exceed  }i\  the  injury  to  the  definition  is  sub- 
stantially negligible  and  virtually  disappears  when  the  image  is 
locussed  for  the  best  definition,  the  loss  of  maximum  intensity 
in  the  star  disc  amounting  to  less  than  20%. 

Even  twice  this  error  is  not  a  very  serious  matter  and  can  be 
for  the  most  part  compensated  by  a  minute  change  of  focus  as  is 
very  beautifully  shown  in  a  paper  by  Buxton(  M.  N.  81,  547), 
which  should  be  consulted  for  detail  of  the  variations  to  be 
effected. 

Conrady  finds  a  given  change  dp  in  the  difference  in  lengths  of 
the  optical  paths,  related  to  the  equivalent  linear  change  of 
focus,  df,  as  follows: — 


df  =  8dp  (^y 


A  being  the  aperture  and  /  the  focal  length,  which  indicates  for 
telescopes  of  ordinary  focal  ratio  a  tolerance  of  the  order  of 
±0.01  inch  before  getting  outside  the  limit  I'iX  for  variation  of 
path. 

For  instruments  of  greater  relative  aperture  the  precision  of 
focus  and  in  general  the  requirements  for  lessened  aberration 
are  far  more  severe,  proportional  in  fact  to  the  square  of  this 
aperture.  Hence  the  severe  demands  on  a  reflector  for  exact 
figure.  An  instrument  working  at  F/5  or  F/6  is  extremely  sensi- 
tive to  focus  and  demands  great  precision  of  figure  to  fall  within 
permissible  values,  say  yi'k  to  j-^X,  for  dp. 

Further,  with  a  given  value  of  dp  and  the  relation  established 

/ 
by  the  chromatic  aberration,  i.e.,  about  ^^k?^'  a  relation  is  also 

determined  between  /  and  A,  required  to  bring  the  aberration 
within  limits.     The  equation  thus  found  is 

/  =  2.8A2 

This  practically  amounts  to  the  common  F/15  ratio  for  an  aper- 
ture of  approximately  5  inches.     For  smaller  apertures  a  greater 


SEEING  AND  MAGNIFICATION  267 

ratio  can  be  well  used,  for  larger,  a  relatively  longer  focus  is 
indicated,  the  penalty  being  light  spread  into  a  halo  over  the 
diffraction  image  and  reducing  faint  contrasts  somewhat 
seriously. 

This  is  one  of  the  factors  aside  from  atmosphere,  interfering 
with  the  full  advantage  of  large  apertures  in  refractors.  While  as 
already  noted  small  amounts  of  spherical  aberration  may  be  to  a 
certain  extent  focussed  out,  the  sign  of  df  must  change  with  the 
sign  of  the  residual  aberration,  and  a  quick  and  certain  test  of  the 
presence  of  spherical  aberration  is  a  variation  in  the  appearance 
of  the  image  inside  and  outside  focus. 

To  emphasize  the  importance  of  exact  knowledge  of  existing 
aberrations  note  Fig.  187,  which  shows  the  results  of  Hartmann 
tests  on  a  typical  group  of  the  world's  large  objectives.  All 
show  traces  of  residual  zones,  but  differing  greatly  in  magnitude 
and  position  as  the  attached  scales  show.  The  most  conspicuous 
aberrations  are  in  the  big  Potsdam  photographic  refractor,  the 
least  are  in  the  24  inch  Lowell  refractor.  The  former  has  since 
been  refigured  by  Schmidt  and  revised  data  are  not  yet  available; 
the  latter  received  its  final  figure  from  the  Lundins  after  the  last 
of  the  Clarks  had  passed  on. 

Now  a  glance  at  the  curves  shows  that  the  bad  zone  of  the 
Potsdam  glass  was  originally  near  the  periphery,  (I),  hence  both 
involved  large  area  and,  from  Conrady's  equation,  seriously 
enlarged  df  due  to  the  large  relative  aperture  at  the  zone.  An 
aberrant  zone  near  the  axis  as  in  the  stage  (III)  of  the  Potsdam 
objective  or  in  the  Ottawa  15  inch  objective  is  much  less  harmful 
for  corresponding  reasons.  Such  differences  have  a  direct  bearing 
on  the  use  of  stops,  since  these  may  do  good  in  case  of  peripheral 
aberration  and  harm  when  the  faults  are  axial.  Unless  the 
aberrations  are  known  no  general  conclusions  can  be  drawn  as 
to  the  effect  of  stops.  Even  in  the  Lowell  telescope  shown  as  a 
whole  in  Fig.  188,  the  late  Dr.  Lowell  found  stops  to  be  useful 
in  keeping  down  atmospheric  troubles  and  reducing  the  illumina- 
tion although  they  could  have  had  no  effect  in  relation  to  figure. 
Fig.  188  shows  at  the  head  of  the  tube  a  fitting  for  a  big  iris 
diaphragm,  controlled  from  the  eye-end,  the  value  of  which  was 
well  demonstrated  by  numerous  observers. 

There  are,  too,  cases  in  which  a  small  instrument,  despite 
intrinsic  lack  of  resolving  power,  may  actually  do  better  work 
than  a  big  one.     Such  are  met  in  instances  where  extreme  con- 


268 


THE  TELESCOPE 


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SEEING  AND  MAGNIFICATION  269 

trast  of  details  is  souglit,  as  has  been  well  pointed  out  by  Nutting 
(Ap.  J.  40,  33)  and  the  situation  disclosed  by  him  finds  amplifica- 
tion in  the  extraordinary  work  done  by  Barnard  with  a  cheap 
lantern  lens  of  13^^  inch  diameter  and  b^'i  inches  focus  (Pop.  Ast., 
6,  452). 

The  fact  is  that  every  task  must  seek  its  own  proper  instru- 
ment. And  in  any  case  the  interpretation  of  observed  results 
is  a  matter  that  passes  far  beyond  the  bounds  of  geometrical 
optics,  and  involves  physiological  factors  that  are  dominant  in 
all  visual  problems. 

With  respect  to  the  visibility  of  objects  the  general  diffrac- 
tion theory  again  comes  into  play.  For  a  bright  line,  for 
example,  the  diffraction  figure  is  no  longer  chiefly  a  cone  like 
Fig.  183,  but  a  similar  long  wedge-shaped  figure,  with  wave-like 
shoulders  corresponding  to  the  diffraction  rings.  The  visibility 
of  such  a  line  depends  not  only  on  the  distribution  of  intensity  in 
the  theoretical  wedge  but  on  the  sensitiveness  of  the  eye  and  the 
nature  of  the  background  and  so  forth,  just  as  in  the  case  of  a 
star  disc. 

If  the  eye  is  from  its  nature  or  state  of  adaptation  keen  enough 
on  detail  but  not  particularly  sensitive  to  slight  differences  of 
intensity,  the  line  will  very  likely  be  seen  as  if  a  section  were 
made  of  the  wedge  near  its  thin  edge.  In  other  words  the 
line  will  appear  thin  and  sharp  as  the  diffraction  rings  about  a 
star  frequently  do. 

With  an  eye  very  sensitive  to  light  and  small  differences  of 
contrast  the  appearance  of  absolutely  the  same  thing  may 
correspond  to  a  section  through  the  wedge  near  its  base,  in  other 
words  to  a  broad  strip  shading  off  somewhat  indistinctly  at  the 
edges,  influenced  again  by  irradiation  and  the  character  of  the 
background. 

If  there  be  much  detail  simultaneously  visible  the  diffraction 
patterns  may  be  mixed  up  in  a  most  intricate  fashion  and  one 
can  readily  see  the  confusion  which  may  exist  in  correlating 
the  work  of  various  observers  on  things  like  planetary  and  lunar 
detail. 

In  the  planetary  case  the  total  image  is  a  complex  of  illumin- 
ated areas  of  diffraction  at  the  edges,  which  may  be  represented 
as  the  diffraction  sohd  of  Fig.  185,  in  which  the  dotted  lines  show 
what  may  correspond  fairly  to  the  real  diameter  of  the  planet, 
the  edge  shading  off  in  a  way  again  complicated  by  irradiation. 


270 


THE  TELESCOPE 


Fig.   188.— The  Lowell  Refractor  Fitted  with  Iris  Diaphragm. 


SEEING  AND  MAGNIFICATION  271 

Fancy  detail  superimposed  on  a  disc  of  this  sort  and  one  has  a 
vivid  idea  of  the  difficulty  of  interpreting  observations. 

It  would  be  an  exceedingly  good  thing  if  everyone  who  uses 
his  telescope  had  the  advantage  of  at  least  a  brief  course  in 
microscopy,  whereby  he  would  gain  very  much  in  the  practical 
understanding  of  resolving  power,  seeing  conditions,  and  the 
interpretation  of  the  image.  The  principles  regarding  these 
matters  are  in  fact  very  much  the  same  with  the  two  great  instru- 
ments of  research. 

Aperture,  linear  in  the  case  of  the  telescope  and  the  so-called 
numerical  in  the  case  of  the  microscope,  bear  precisely  the  same 
relation  to  resolution,  the  minimum  resolvable  detail  being  in 
each  case  directly  proportional  to  aperture  in  the  senses  here 
employed. 

Further,  although  the  turbulence  of  intervening  atmosphere 
does  not  interfere  with  the  visibility  of  microscopic  detail,  a 
similar  disturbing  factor  does  enter  in  the  form  of  irregular  and 
misplaced  illumination.  It  is  a  perfectly  easy  matter  to  make 
beautifully  distinct  detail  quite  vanish  from  a  microscopic  image 
merely  by  mismanagement  of  the  illumination,  just  as  unsteady 
atmosphere  will  produce  substantially  the  same  effect  in  the 
telescopic  image. 

In  the  matter  of  magnification  the  two  cases  run  quite  parallel, 
and  magnification  pushed  beyond  what  is  justified  by  the  resolv- 
ing power  of  the  instrument  does  substantially  little  or  no  good. 
It  neither  discloses  new  detail  nor  does  it  bring  out  more  sharply 
detail  which  can  be  seen  at  all  with  a  lower  power. 

The  microscopist  early  learns  to  shun  high  power  oculars, 
both  from  their  being  less  comfortable  to  work  with,  and  from 
their  failing  to  add  to  the  efficiency  of  the  instrument  except  in 
some  rare  cases  with  objectives  of  very  high  resolving  power. 
Furthermore  in  the  interpretation  of  detail  the  lessons  to  be 
learned  from  the  two  instruments  are  quite  the  same,  although 
one  belongs  to  the  infinitely  little  and  the  other  to  the  infinitely 
great. 

Nothing  is  more  instructive  in  grasping  the  relation  between 
resolving  power,  magnification,  and  the  verity  of  detail,  than  the 
study  under  the  microscope  of  some  well  known  objects.  For 
example,  in  Fig.  189  is  shown  a  rough  sketch  of  a  common  diatom, 
Navicula  Lyra.  The  tiny  siliceous  valve  appears  thus  under  an 
objective  of  slightly  insufficient  resolving  power.     The  general 


272 


THE  TELESCOPE 


form  of  the  object  is  clearly  perceived,  as  well  as  the  central 
markings,  standing  boldly  out  in  the  form  which  suggests  the 
specific  name.  No  trace  of  any  finer  detail  appears  and  no 
amount  of  dexterity  in  arranging  the  illumination  or  increase  of 
magnifying  power  will  show  any  more  than  here  appears,  the 
drawing  beng  one  actually  made  with  the  camera  lucida,  using 
an  objective  of  numerical  aperture  just  too  small  to  resolve  the 
details  of  the  diatoms  on  this  particular  slide. 

Figure  189a  shows  what  happens  when,  with  the  same  magnify- 
ing power,  an  objective  of  slightly  greater  aperture  is  employed. 
Here  the  whole  surface  of  the  valve  is  marked  with  fine  stria- 


FiG.   189. — The  Stages  of  Resolution. 

tions,  beautifully  sharp  and  distinct  like  the  lines  of  a  steel 
engraving.  There  is  a  complete  change  of  aspect  wrought  by  an 
increase  of  about  20%  in  the  resolving  power.  Again  nothing 
further  can  be  made  out  by  an  increase  of  magnification,  the 
only  effect  being  to  make  the  outlines  a  little  hazier  and  the  view 
therefore  somewhat  less  satisfactory. 

Finally  in  Fig.  189&  we  have  again  the  same  valve  under  the 
same  magnifying  power,  but  here  obtained  from  an  objective 
of  numerical  aperture  60%  above  that  used  for  the  main  figure. 
The  sharp  striae  now  show  their  true  character.  They  had  their 
origin  in  lines  of  very  clearly  distinguished  dots,  which  are 
perfectly  distinct,  and  are  due  to  the  resolving  power  at  last  being 
sufficient  to  show  the  detail  which  previously  merely  formed  a 
sharp  linear  diffraction  pattern  entirely  incapable  of  being  re- 
solved into  anything  else  by  the  eye,  however  much  it  might 
be  magnified. 


SEEING  AND  MAGNIFICATION  273 

Here  one  has,  set  out  in  unmistakable  terms,  the  same  kind 
of  differences  which  appear  in  viewing  celestial  detail  through 
telescopes  of  various  aperture.  What  cannot  be  seen  at  all  with 
a  low  aperture  may  be  seen  with  higher  ones  under  totally  differ- 
ent aspects;  while  in  each  case  the  apparent  sharpness  and 
clarity  of  the  image  is  somewhat  extraordinary. 

Further  in  Fig.  1896  in  using  the  resolving  power  of  the  objec- 
tive of  high  numerical  aperture,  the  image  may  be  quite  wrecked 
by  a  little  carelessness  in  focussing,  or  by  mismanagement  of 
light,  so  that  one  would  hardly  know  that  the  valve  had  markings 
other  than  those  seen  with  the  objectives  of  lower  aperture,  and 
under  these  circumstances  added  magnification  would  do  more 
harm  than  good.  In  precisely  the  same  way  mismanagement  of 
the  illumination  in  Fig.  189a  would  cause  the  striae  to  vanish  and 
with  Navicula  Lyra,  as  with  many  other  diatoms,  the  resolution 
into  striae  is  a  thing  which  often  depends  entirely  on  careful 
lighting,  and  the  detail  flashes  into  distinctness  or  vanishes  with  a 
suddenness  which  is  altogether  surprising.  For  "lighting" 
read  "atmosphere,"  and  you  have  just  the  sort  of  conditions 
that  exist  in  telescope  vision. 

With  respect  to  magnifying  powers  what  has  already  been 
said  is  sufficient  to  indicate  that  on  the  whole  the  lowest  power 
which  discloses  to  the  eye  the  detail  within  the  reach  of  the 
resolving  power  of  the  objective  is  the  most  satisfactory. 

Ever}'  increase  above  this  magnifies  all  the  optical  faults  of 
the  telescope  and  the  atmospheric  difficulties  as  well,  beside 
decreasing  the  diameter  of  the  emergent  pencil  which  enters  the 
eye,  and  thereby  causing  serious  loss  of  acuity.  For  the  eye 
like  any  other  optical  instrument  loses  resolving  power  with 
decrease  of  effective  aperture,  and,  besides,  a  very  narrow  beam 
entering  it  is  subject  to  the  interference  of  entoptic  defects,  such 
as  floating  motes  and  the  like,  to  a  serious  extent. 

Figure  190  shows  from  Cobb's  experiments  (Am.  Jour,  of  Physiol, 
35,  335)  the  effect  of  reduction  of  ocular  aperture  upon  acuity. 
The  curve  shows  very  plainly  that  for  emergent  pencils  below  a 
millimeter  (3^-^  5  inch)  in  diameter,  visual  acuity  falls  off  almost 
in  direct  proportion  to  the  decreasing  aperture.  Below  this  figure 
there  can  be  only  incidental  gains,  such  as  may  be  due  to  opening 
up  double  stars  and  simultaneously  so  diminishing  the  general 
illumination  as  to  render  the  margins  of  the  star  discs  a  little 
less  conspicuous. 


274 


THE  TELESCOPE 


An  emergent  pencil  of  this  diameter  is  not  quite  sufficient  for 
the  average  eye  to  utiUze  fully  the  available  resolving  power  and 
some  excess  of  magnification  even  though  it  actually  diminishes 
visual  acuity  materially,  may  be  of  some  service. 

Increased  acuity  will  of  course  be  gained  for  the  same  magni- 
fication in  using  an  objective  of  greater  diameter,  to  say  nothing 


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Fig.   190. — Resolving  Power  of  the  Eye. 

of  increased  resolving  power,  at  the  cost,  of  course,  of  relatively 
greater  atmospheric  troubles. 

To  come  down  to  figures  as  to  the  resolving  power  of  the  eye, 
often  repeated  experiments  have  shown  that  two  points  offering 
strong  contrast  with  the  background  can  be  noted  as  separate 
by  the  normal  eye  when  at  an  angular  separation  of  about  3' 
of  arc.  People,  as  we  have  seen,  differ  considerably  in  acuity 
so  that  now  and  then  individuals  will  considerably  better  this 
figure,  while  others,  far  less  keen  sighted,  may  require  a  separa- 
tion of  4'  or  even  5'. 

The  pair  of  double  stars  ei,  €2,  Lyrse,  separated  by  3'  27' 
mags,    nearly   4    and   5  respectively,   can  be  seen  as  separate 


SEEING  AND  MAGNIFICATION  275 

by  those  of  fairly  keen  vision,  while  Mizar  and  Alcor,  11' 
apart,  seem  thrown  wide  to  nearly  every  one.  On  the  other 
hand  the  writer  has  never  known  anybody  who  could  separate 
the  two  components  of  Asterope  of  the  Pleiades,  distant  a  scant 
23^-2  but  of  mags.  6.5  and  7.0  only,  while  Pleione  and  Atlas, 
distance  about  53^^',  mags.  6.5  and  4,  are  very  easy. 

Assuming  for  liberality  that  the  separation  constant  is  in  the 
neighborhood  of  5'  one  can  readily  estimate  the  magnifica- 
tion that  for  any  telescope  will  take  full  advantage  of  its  resolving 
power.     As  we  have  already  seen  this  resolving  power  is  practi- 

4.  "56 
cally — 2 —  ^o^  equal  stars  moderately  bright.     An  objective  of 

4.56'  inches  aperture  has  a  resolving  constant  of  1'^  and  to 
develop  this  should  take  a  magnification  of  say  300,  about  65 
to  the  inch  of  aperture,  requiring  a  focal  length  of  ocular  about 
0.20  to  0.25  inch  for  telescopes  of  normal  relative  aperture,  and 
pushing  the  emergent  pencil  down  to  little  more  than  0.02  inch, — 
rather  further  than  is  physiologically  desirable.  Except  for  these 
extreme  stunts  of  separation,  half  to  two  thirds  this  power  is 
preferable  and  conditions  under  which  one  can  advantageously 
go  above  this  limit  are  very  rare  indeed. 

A  thoroughly  good  objective  or  mirror  will  stand  quite  100 
magnification  to  the  inch  without,  as  the  microscopist  would  say, 
"breaking  down  the  image,"  but  in  at  least  nine  cases  out  of  ten 
the  result  will  be  decidedly  unsatisfactory. 

As  the  relative  aperture  of  the  instrument  increases,  other 
things  being  equal,  one  is  driven  to  oculars  of  shorter  and  shorter 
focus  to  obtain  the  same  magnification  and  soon  gets  into  trouble. 
Very  few  oculars  below  0.20  inch  in  focus  are  made,  and  such 
are  rarely  advisable,  although  occasionally  in  use  down  to  0.15 
inch  or  thereabouts.  The  usual  F/15  aperture  is  a  figure  quite 
probably  as  much  due  to  the  undesirability  of  extremely  short 
focus  oculars  as  to  the  easier  corrections  of  the  objective. 

In  the  actual  practice  of  experienced  observers  the  indications 
of  theory  are  well  borne  out.  Data  of  the  habits  of  many  observ- 
ers of  double  stars  are  of  record  and  the  accomplished  veteran 
editor  of  The  Observatory ,  Mr.  T.  Lewis,  took  the  trouble  in  one 
of  his  admirable  papers  on  "Double  Star  Astronomy"  (Obs.  36, 
426)  to  tabulate  from  the  original  sources  the  practice  of  a  large 
group  of  experts.  The  general  result  was  to  show  the  habitual 
use  with  telescopes  of  moderate  size  of  powers  around  50  per 


276  THE  TELKHCOPK 

inch  of  aperture,  now  and  then  on  special  occasions  raised  to  the 
neighborhood  of  70  per  inch. 

But  the  data  showed  unequivocally  just  what  has  been  already 
indicated,  that  large  apertures,  suffering  severely  as  they  gener- 
ally do  from  turbulence  of  the  air,,  will  not  ordinarily  stand  their 
due  proportion  of  magnification.  With  the  refractors  of  24  inches 
aperture  and  upwards  the  records  show  that  even  in  this  double 
star  work,  where,  if  anywhere,  high  power  counts,  the  general 
practice  ran  in  the  vicinity  of  30  per  inch  of  aperture. 

Analyzing  the  data  more  completely  in  this  respect  Mr. 
Lewis  found  that  the  best  practise  of  the  skilled  observers  studied 
was  approximately  represented  by  the  empirical  equation 

m  =  140\/A 

Of  course  the  actual  figures  must  vary  with  the  conditions  of 
location  and  the  general  quality  of  the  seeing,  as  well  as  the 
work  in  hand.  For  other  than  double  star  work  the  tendency  will 
be  generally  toward  lower  powers.  The  details  which  depend 
on  shade  perception  rather  than  visual  acuity  are  usually  hurt 
rather  than  helped  when  magnified  beyond  the  point  at  which 
they  are  fairly  resolved,  quite  as  in  the  case  of  the  microscope. 

Now  and  then  they  may  be  made  more  distinct  by  the  judicious 
use  of  shade  glasses.  Quite  apart  from  the  matter  of  the  high 
powers  which  can  advantageously  be  used  on  a  telescope,  one 
must  for  certain  purposes  consider  the  lowest  powers  which  are 
fairly  applicable.  This  question  really  turns  on  the  largest 
utilizable  emergent  pencil  from  the  eye  piece.  It  used  to  be 
commonly  stated  that  3^^  inch  for  the  emergent  pencil  was 
about  a  working  maximum,  leading  to  a  magnification  of  8  per 
inch  of  aperture  of  the  objective.  This  in  view  of  our  present 
knowledge  of  the  eye  and  its  properties  is  too  low  an  estimate  of 
pupillary  aperture.  It  is  a  fact  which  has  been  well  known  for 
more  than  a  decade  that  in  faint  light,  when  the  eye  has  become 
adapted  to  its  situation,  the  pupil  opens  up  to  two  or  three  t^mes 
this  diameter  and  there  is  no  doubt  that  a  fifth  or  a  fourth  of  an 
inch  aperture  can  be  well  utilized,  provided  the  eye  is  properly 
dark-adapted.  For  scrutinizing  faint  objects,  comet  sweeping 
and  the  like,  one  should  therefore  have  one  ocular  of  very  wide 
field  and  magnifying  power  of  4  or  5  per  inch  of  aperture,  the 
main  point  being  to  secure  a  field  as  wide  is  practicable.  One 
may  use  for  such  purposes  either  a  very  wide  field  Huyghenian, 


SEEING  AND  MAGNIFICATION  277 

or,  if  cross  wires  are  to  be  used,  a  Kellner  form.  Fifty  degrees  of 
field  is  perfectly  practicable  with  either.  As  regards  the  rest 
of  the  eye-piece  equipment  the  observer  may  well  suit  his  own 
convenience  and  resources.  Usually  one  ocular  of  about  half 
the  maximum  power  provided  will  be  found  extremely  convenient 
and  perhaps  oftener  used  than  either  the  high  or  low  power. 
Oculars  of  intermediate  power  and  adapted  for  various  purposes 
will  generally  find  their  way  into  any  telescopic  equipment.  And 
as  a  last  word  do  not  expect  to  improve  bad  conditions  by 
magnifying.  If  the  seeing  is  bad  with  a  low  power,  cap  the 
telescope  and  await  a  better  opportunity. 


APPENDIX 
WORK  FOR  THE  TELESCOPE 

To  make  at  first  hand  the  acquaintance  of  the  celestial  bodies 
is,  in  and  of  itself,  worth  the  while,  as  leading  the  mind  to  a 
new  sense  of  ultimate  values.  To  tell  the  truth  the  modern  man 
on  the  whole  knows  the  Heavens  less  intimately  than  did  his 
ancestors.  He  glances  at  his  wrist-watch  to  learn  the  hour  and 
at  the  almanac  to  identify  the  day.  The  rising  and  setting  of 
the  constellations,  the  wandering  of  the  planets  among  the 
stars,  the  seasonal  shifting  of  the  sun's  path — all  these  are  a 
sealed  book  to  him,  and  the  intricate  mysteries  that  lie  in  the 
background  are  quite  unsuspected. 

The  telescope  is  the  lifter  of  the  cosmic  veil,  and  even  for 
merely  disclosing  the  spectacular  is  a  source  of  far-reaching 
enlightenment.  But  for  the  serious  student  it  offers  oppor- 
tunities for  the  genuine  advancement  of  human  knowledge 
that  are  hard  to  underestimate.  It  is  true  that  the  great  modern 
observatories  can  gather  information  on  a  scale  that  staggers 
the  private  investigator.  But  in  this  matter  fortune  favors 
the  pertinacious,  and  the  observer  who  settles  to  a  line  of  delib- 
erate investigation  and  patiently  follows  it  is  likely  to  find  his 
reward.  There  is  so  much  within  the  reach  of  powerful  instru- 
ments only,  that  these  are  in  the  main  turned  to  their  own 
particular  spheres  of  usefulness. 

For  modest  equipment  there  is  still  plenty  of  work  to  do. 
The  study  of  variable  stars  offers  a  vast  field  for  exploration, 
most  fruitful  perhaps  with  respect  to  the  irregular  and  long- 
period  changes  of  which  our  own  Sun  offers  an  example.  Even 
in  solar  study  there  are  transient  phenomena  of  sudden  erup- 
tions and  of  swift  changes  that  escape  the  eye  of  the  spectro- 
heliograph,  and  admirable  work  can  be  done,  and  has  been  done, 
with  small  telescopes  in  studying  the  spectra  of  sun  spots 

Temporary  stars  visible  to  the  naked  eye  or  to  the  smallest 
instruments  turn  up  every  few  years  and  their  discovery  has 
usually  fallen  to  the  lot  of  the  somewhat  rare  astronomer, 
professional  or  amateur,  who  knows  the  field  of  stars  as  he  knows 

279 


280  APPENDIX 

the  alphabet.  The  last  three  important  novae  fell  to  the  ama- 
teurs— two  to  the  same  man.  Comets  are  to  be  had  for  the 
seeking  by  the  persistent  observer  with  an  instrument  of  fair 
light-grasp  and  field;  one  distinguished  amateur  found  a  pair 
within  a  few  days,  acting  on  the  theory  that  small  comets  are 
really  common  and  should  be  looked  for — most  easily  by  one 
who  knows  his  nebulae,  it  should  be  added. 

And  within  our  small  planetary  system  lies  labor  sufficient 
for  generations.  We  know  little  even  about  the  superficial 
characters  of  the  planets,  still  less  about  their  real  physical 
condition.  We  are  not  even  sure  about  the  rotation  periods  of 
Venus  and  Neptune.  The  clue  to  many  of  the  mysteries  requires 
eternal  vigilance  rather  than  powerful  equipment,  for  the 
appearance  of  temporary  changes  may  tell  the  whole  story. 
The  old  generation  of  astronomers  who  believed  in  the  complete 
inviolability  of  celestial  order  has  been  for  the  most  part  gathered 
to  its  fathers,  and  we  now  realize  that  change  is  the  law  of  the 
universe.  Within  the  solar  system  there  are  planetary  surfaces 
to  be  watched,  asteroids  to  be  scanned  for  variability  or  change 
of  it,  meteor  swarms  to  be  correlated  with  their  sources,  occupa- 
tions to  be  minutely  examined,  and  when  one  runs  short  of  these, 
our  nearest  neighbor  the  Moon  offers  a  wild  and  physically 
unknown  country  for  exploration.  It  is  suspected  with  good 
reason  of  dynamic  changes,  to  say  nothing  of  the  possible  last 
remnants  of  organic  life. 

Much  of  this  work  is  well  within  the  useful  range  of  instruments 
of  three  to  six  inches  aperture.  The  strategy  of  successful 
investigation  is  in  turning  attention  upon  those  things  which  are 
within  the  scope  of  one's  equipment,  and  selecting  those  which 
give  promise  of  yielding  to  a  well  directed  attack.  And  to  this 
end  efforts  correlated  with  those  of  others  are  earnestly  to  be 
advised.  It  is  hard  to  say  too  much  of  the  usefulness  of  directed 
energies  like  those  of  the  Variable  Star  Association  and  similar 
bodies.  They  not  only  organize  activities  to  an  important 
common  end,  but  strengthen  the  morale  of  the  individual 
observer. 


INDEX 


Abbe,  roof  prism,  162 
Aberration,  compensated  by  minute 
change  of  focus,  266 

illuminates       the       diffraction 
minima,  265 

relation    determines    of    focus 
and  aperture,  266 
Achromatic  long  relief  ocular,    146 

objective,  77 
Achromatism,  condition  for,  78 

determination  of,  78 

imperfection  of,  87 
Adjustment  where  Polaris  invisible, 

235 
Air  waves,  length  of,  255 
Alt-azimuth  mount  for  reflector,  102 

mounts,  with  slow  motions,  102 

setting  up  an,  228 
Anastigmats,  84 
Annealing,  pattern  of  strain,  68 
Astigmatism,  84,  209 

of  figure,  210 
Astronomy,  dawn  of  popular,  19 

B 

Bacon,  Roger,  alleged  description  of 

telescopes,  6 
Barlow  lens,  152 
"Bent,"  objective,  86 
Binocular,  2 

advantage  of,  exaggerated,  151 
for    strictly    astronomical    use, 

152 
telescopes  for  astronomical  use, 
163 

C 

Camouflage,  in  optical  patents,  97 
Cassegrain,     design     for     reflecting 
telescope,  22 


Cassegrain,  sculptor  and  founder  of 

statues,  22 
Cell,  taking  off  from  a  telescope,  202 
Chromatic  aberration,  11,  76 
investigation  of,  210 
correction,  differences  in,  91 
error  of  the  eye,  90 
Clairault's  condition,  81 

two  cemented  forms  for,  81 
Clarks,  portable  equatorial 

mounting,  109 
terrestrial    prismatic    eyepiece, 
158 
Clock,  the  cosmic,  233 
Clock  drive,  110,  174 
Clock    mechanism,    regulating    rate 

of  motor,  179 
Coddington  lens,  137 
Coelostat  constructions,  126 

tower  telescopes,  127 
Color    correction,    commonly    used, 
211 
examined    by     spectroscope, 

211 
of  the  great  makers,  90 
Coma-free,       condition       combined 

with  Clairault's,  83 
Comet   seeker,    Caroline   Herschel's 
118 
seekers    with    triple    objective, 
119 
Crowns    distinguished    from    flints, 

64 
Curves,   struggle  for  non-spherical, 
18 


D 


Davon  micro-telescope,  148 
Dawes'  Limit,  261 

in  physiological  factors,  263 
Declination  circle,  108 

adjustment  of,  239 


281 


282 


INDEX 


Decimation   circle,   adjustment    by, 
237 
facilitates  setting  up  instru- 
ment, 110 
Definition   condition   for   excellence 
of,  254 
good  in  situations  widely  dif- 
ferent, 254 
DeRheita,  12 

constructed  binoculars,  13 
terrestrial  ocular,  13 
Descartes'  dioptrics,  publication  of, 
11 
lens  with  elliptical  curvature,  12 
Dew  cap,  219 

Diaphragms,  importance  of,  43 
Diffraction  figure  for  bright  line,  269 
pattern,  256 

solid,  apparent  diameter  of,  262 
solid  of  planet,  269 
solid  for  a  star,  260 
spectra,  190 
system,  scale  of,  260 

varies  inversely  with 

aperture,  260 
through  objective,  258 
Digges,    account    suggests    camera 

obscura,  7 
Dimensions,     customary,     telescope 

of,  24 
Discs,  inspection  of  glass,  66 

roughing  to  form,  69 
Distortion,  86 
DoUand,  John,  28 

published    his    discovery    of 
achromatism,  29 
Peter,  early  triple  objective,  29 
Dome  wholly  of  galvanized  iron,  250 
Domes,  246 
Driving  clock,  a  simple,  174 

pendulum  controlled,  177 
clocks  spring  operated,  175 


E 


English  equatorial,  110 

mounts,  mechanical  stability  of, 
113 
Equatorial,  adjustments  of,  230 


Equatorial,  coude,  124 

mount,    different    situations   in 

using,  229 
mount,  first  by  Short,  104 
mount,  pier  overhung,  115 
mount  in  section,  107 
two  motions  necessary  in,   106 
Equilibrating     levers,     devised     by 

T.  Grubb,  39 
Evershed,        direct        vision    solar 

spectroscope,  189 
Eye  lens,  simple,  preferred  by  Sir  W. 

Herschel,  136 
Eyepiece,  compensating,  142 
Huygenian,  139 
Huygenian,     achromatism     of, 

140 
Huygenian,    with    cross  wires, 

140 
Huygenian,  field  of,  141 
Huygenian     focal     length     of, 

143 
measuring  focus  of,  136 
microscope  form,  147,  148 
monocentric,  139 
a  simple  microscope,  134 
ToUes  soUd,  141 


Field,  curvature  of,  85 

glass,  arrangement  of  parts,  151 
Galilean,  150 

lens    diameter    possible,    150 
Field  lens,  139 
Figuring  locally,  73 

process  of,  73 
Filar  micrometer,  172 
Finder,  108,  132 

adjustment  of,  230 
Fine  grinding,  69 
Fixed  eyepiece  mounts,  118 
Flints,     highly    refractive    due    to 

Guinand,  36 
Foucault,  39 

development  of  silver  on  glass 

reflector,  41 
knife  edge  test,  212 


INDEX 


283 


Foucault,   methods  of  working  and 

testing,  41 
Fraunhofer,  36 

applied    condition    of    absence 

of  coma,  82 
form  of  objectives,  37 
long    list    of    notable    achieve- 
ments, 38 
"Front  view"  telescope,  32 

mechanical  difficulty  of,  33 
Furnaces,  glass,  classes  of,  59 

G 

Galilean  telescope,  small  field  of,  9 
Galileo,      exhibited      telescope      to 
senators  of  Venice,  8 
grasps  the  general  principles,  7 
produces   instrument    magnify- 
ing 32  times,  8 
Gascoigne,      William,      first      using 

genuine  micrometer,  12 
Gauss,  Objective,  82 
Gerrish,    application    of   drive,    181 

motor  drive,  179 
Ghosts,  137 

Glass,  dark,  as  sunshade,  166 
forming  and  annealing,  62 
inspection  of  raw,  61 
losses   by   volatilization,    58 
materials  of,  59 
origin  of,  57 

persistent  bubbles  in,  58 
a  solid  solution,  57 
Grating  spectroscopes,  190 
Gratings,  spectroscope,  189 
Gregory,  James,  described  construc- 
tion which  bears  his  name, 
19 
failed    of   material   success,    20 
Grubb,  Sir  Howard,  objectives,  74 
Guinand,     Pierre    Louis,    improve- 
ments in  optical  glass,  36 

H 

Hadley,     disclosed    test    for    true 
figure,  27 
John,  real  inventor  of  reflector, 
25 


Hadley's  reflector,  tested  with  satis- 
factory results,  26 
Hall,   Chester  Moor,   designed  first 
achromatic  telescope,  27 
had  telescopes  made  as  early  as 
1733,  27 
Hand  telescope,  magnifying  power, 
150 
monocular,  151 
Hartmann  test,  213 

on  large  objectives,  267 
principle  of,  214 
Hartness,  turret  telescope,  130,  131 
Heliometer,  principle  of,  171 
Hensoldt,  prism  form,  163 
Herschel's  discovery  of  Uranus,  32 
forty  foot  telescope,  34 
Sir  John,  35 
Sir     John,     proposed     defining 

condition,  81 
Sir  Wilham,  31 
Herschel's  time,  instruments  of,  35 
Hevelius,  construction  for  objective 
of  150  feet,  17 
directions  for  designing  Galilean 
and  Keplerian     telescopes, 
14 
invention  of  first  periscope,  15 
Johannes,  13 
mention  of  advantage  of  piano 

convex  lens,  14 
mentions      telescope      due      to 
DeRheita,  14 
Housing      reflector      of      36      inch 
aperture,  243 
rolling  on  track,  242 
simplest   instrument   for   fixed, 
241 
Huygens,         Christian,         devised 
methods     of     grinding     & 
polishing,  16 
Huygens'  eyepiece,  introduction  of, 

24 
Huygens,  sketch  of  Mars,  16 


Image,     correct     extra     focal,     208 
critical  examination  of,  204 


284 


INDEX 


Imago,  curvature  of,  87 

seen  without  eyepiece,  134 
showing    unsymmetrical    color- 
ing, 208 

Interference    rings,    eccentric,    205 

Irradiation,  262 


Jansen,  Zacharius,  4 


K 


Kellner,  ocular,  145 
Kepler,  astronomical  telescope,  10 
differences     of    from     Galilean 

form,  10 
Knife  edge  test  of  parabolic  mirror, 

212 


Lacquer,  endurance  of  coating,  223 
Latitude  scale,  232 
Lenses,    determinate   forms   for,    80 
Lens,    magnifying    power    of,     134 

"crossed,"  24 

polishing    the   fine    ground,    70 

power  of,  78 

triple  cemented,  a  useful  ocular, 
138 

simple  achromatic,  137 

single,  has  small  field,  137 

spotted,  cleaning  of,  217 
Light  grasp  and  resolving  power,  265 
small  telescope  fails  in,  264 
Light  ratio  of  star  magnitudes,  264 
Light  transmitted  by  glass,  53 
Lippershey,  Jan,  2 

discovery,  when  made,  5 

retainer  to,  3 
Lunette  d,  Napoleon  Troisieme,  154, 
155,  162 


M 


Magnifying  power,  directly  as  ratio 
of  increase  in  tangent,  135 
powers,  increase  of,  273 


Marius,  Simon,  5 

used        with         glasses     from 
spectacles,  5 
Marius,     picked     up     satellites     of 

Jupiter,  5 
Meridian  photometer,  194 
Metius,  James,  4 
Metius,  tale  of,  4 
Micrometer,  double  image,  171 

square  bar,  171 
Micrometers,  168 
Micrometry,  foundations  of,  12 
Mirror's,  aberrations  of,  92 
adjustment  of,  206 
concave  spherical,  92 
final  burnishing  of,  226 
hyperboloidal,  96 
lacquer  coating  for  surface,  221 
mounting,     by     Browning,     49 
parabolic  oblique,  shows  aber- 
ration, 95 
surface,    prevention    of    injury 
to,  220 
Mittenzwey  ocular,  141 
Mountain    stations,    good    or   very 

bad,  254 
Mounts,  alt-azimuth  and  equatorial, 

98 
Myopia,  glasses  for,  came  slowly,  2 

N 

Navicula  Lyra,  stages  of  resolution 

of,  271 
Newton,  abandoned  parabolic  mir- 
ror, 21 
blunder  in  experiment,  20 
gave  little    information    about 
material    for    mirrors,     23 
Isaac,  attempt  at  a  reflector,  20 
Normal  spectra,  190 


O 


Objective,  adjustable  mount  for,  44 
adjusting  screws  of,  44 
Clark's  form,  83 
cleansing,  203 
examination  of,  202 


INDEX 


285 


Objective,  four-part,  85 

Fraunhofer  flint-ahead,  83 
how  to  clean,  216 
spacers,  to  take  out,  217 
typical  striae  in,  203 
Objective      prism,      photographing 

with,  185,  187 
Objectives,  crown  glass  equiconvex, 
80  over-achromatized,  90 
rated  on  focal  length  for  green 
24 
Observatories,  cost  of  Romsey,  252 
Observatory  at  small  expense,  249 
Romsey,     description    of,     249 
with    simple    sliding    roof,    245 
Observing  box,  229 
Oblique  fork  alt-azimuth,  100 
Ocular,  apparent  angular  field  of,  146 
terrestrial,  147 
ToUes  terrestrial,  147 
typical  form,  45 
Oculars,     radius     of     curvature  of 
image  in,  146 
undesirability    of    short    focus, 
275 
Open  fork  mount,  115 

well   suited   to   big   reflec- 
tors, 117 
Optical  axis,  to  adjust  declination  of, 

238 
Optical  glass,  classes  of,  63 

data  and  analysis  of,  64 
industry,  due  to  single  man, 

36 
production  of,  60 
Orthoscopic  ocular,  145 


Parallactic  mount,  104 

Petition  for  annulment  of  Dolland's 

patent,  29 
photometer,    artificial   star   ZoUner, 
194 

extinction,  198 

photoelectric  cell,  199 

precision   of   astronomical,    199 

selenium  cell,  199 

Zollner,  197 


Photometers,  three  classes  in  stellar, 

193 
"Photo-visual,  objective,"  89 
Pillar-and-claw  stand,  98 
Pillar  mount,  240 
Pitch,  optician's,  71 
Placement     for     tripod     legs,     236 
Polar  and  coude  forms  of  reflector, 
125 
axis,  adjustment  of  by  level,  232 
axis,  alignment  to  meridian,  232 
axis,  setting  with  finder  altitude 

of,  234 
telescope,  119,  122 
Polaris,  hour  angle  of,  233 

a  variable  star,  199 
Polarizing  photometer,  193 
Pole,  position,  234 
Polishing  machine,  70 
surface  of  tool,  72 
tool,  71 
Porro's  second  form,  157 

work,    original    description    of, 
156 
Porta,    description   unintelligible,    7 
Portable  equatorial,  adjustment  of, 
230 
telescopes,    mounting    of,    228 
Porter  polar  reflector,  130 
Position  angle  micrometer  of  Lowell 

Observatory,  173 
Powers,      lowest     practicable,      276 
Prismatic     inversion,     Porro's    first 

form,  155 
Prismatic  inverting  system,  the  first, 

154 
Prisms,  Dove's,  154 
Prism     field     glasses,     stereoscopic 

effect  of,  159 
Prism  glass,  152 

loss  of  light  in,  160 
objectives  of  ,  161 
weak  points  of,  160 

R 

Resolving    constant,    magnification 
to  develop,  275 
power  and  verity  of  detail,  2 
power  of  the  eye,  274 


286 


INDEX 


Reticulated  micrometer,  169 

Reversion  prism,  153 

Right  ascension  circle,  108 

Ring  micrometer,  169 

computation  of  results  of,  170 

Ring  system  faults  due  to  strain,  205 

"Romsey"    observatory    type,    248 

Rack  motion  in  altitude,  100 

Ramsden,  ocular,  144 

Reflection,   coefficient  of,  from   sil- 
vered surface,  54 

Reflector  costs,  55 
cover  for,  242 

development  in  England,  41 
for  astrophysical  work,  56 
light-grasp  of,  53 
relative  aperture  of,  50 
section  of  Newtonian,  45 
skeleton  construction,  49 
suffers  from  scattered  light,  56 
working  field  of,  55 

Refractive  index,  63 

Refractors    and   reflectors,    relative 
advantages  of,  52 
few     made     after     advent     of 

reflector,  27 
in  section,  43 
light  transmission  of,  53 

Refractors,       relative       equivalent 
apertures  of,  54 
tubes  of,  42 

S 

Scheiner,       Christopher,       use      of 
Kepler's  telescope,  11 
devised   parallactic   mount,    11 
Secondary  spectrum,  87 

new  glasses  reducing,  88 
Seeing,  257 

conditions,     for     difference     of 

aperture,  257 
conditions    generally   bad,    253 
standard  scale  of,  256 
true  inwardness  of  bad,  253 
Separating  power,  to  compute,  261 
Short,      James,     mastered     art     of 
figuring  paraboloid,  27 
took  up  Gregorian  construc- 
tion with  success,  27 


Shortened  telescope,  152 
Sights,  on  portable  mount,  229 
Silver  films,  condition  of,  54 
Silvering,  Ludin's  process,  225 
processes,  222 

process.     Dr.     Brashear's,     222 
Sine  condition,  Abb(§'s,  82 
Slit,    spectroscope.  Abbe    type,  184 
Snow  coelostat  telescope,  127 
Solar  diagonal,  166 

eye  piece  diaphragms  in,  168 
early  spectroscopes,  188 
polarizing  eyepiece,  167 
spectroscope,  187 
Spacers,  44,  218 

Spectacle  lenses,  combination  of,  2 
Spectacles  for  presbyopia,  2 

invention  of,  1 
Spectra,    visibility    of    stellar,     183 
Spectro-heliograph,  principle  of,  191 

simple  type  of  Hale's,  191 
Spectroscope,  182 

construction    of    astronomical, 

182 
of  Lowell  refractor,  185 
ocular,  McClean  form,  183 
Specula,  small,  methods  of  support, 

49 
Speculum  metal  composition  of,  24 
Sphenoid  prisms,  158,  163 
Spherical  aberration,  11 
amount  of,  80 
annulling  in  both  directions, 

84 
examination  for,  207 
quick  test  of,  267 
remedy  for,  79 
concave   mirror,    errors    of,    22 
Star,  appearance  of,  204 
artificial,  66,  203 
diagonal,  165 

disc,  apparent  diameter  of,  259 
image  of  reflector,  206 
Steinheil,  achromatic  ocular,  144 
Karl  August,  silvering  specula, 
39 
Striae,  location  of,  67 
Surface,  treatment  of  deterioration 
of,  218 


INDEX 


287 


Taylor,    triplets   with   reduced   sec- 
ondary spectrum,  89 
Telescopes,  choice  and  purchase  of, 
201 
Early  in  1610  made  in  England, 

6 
first,  3 

the  first  astronomical,  9 
improvement  of  early,  11 
lineage  of,  1 
name   devised,  9 
Telescopes,  portable  and  fixed,   108 
1609,  for  sale  in  Paris,  5 
size  and  mounting  of  early,   14 
Telescopic    vision,    discovery    of,    2 
Templets,    designed    curves   of,    69 
Tests   for   strise   and   annealing,    68 
Transparency,    lack    of    in    atmos- 
phere, 255 


Triplet,  cemented,  85 

Turret  housing  of  reflector,  244 


Variable  stars,  192 

W 

Wedge  calibrated  by  .observation, 
197 

photographic,  197 

photometer,  197 
Wind,  shelter  from,  240 

Z 

Zeiss,  binocular  of  extreme  stereo- 
scopic effect,  161 

Zollner,  photometer  modification 
of,  198 

Zonal  aberration,  209 


016 


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